Fix SMP related bugs

* sched/sched: Fix a deadlock in SMP mode

    Two months ago, I introduced sched_tasklist_lock() and
    sched_tasklist_unlock() to protect tasklists in SMP mode.
    Actually, this change works pretty well for HTTP audio
    streaming aging test with lc823450-xgevk.

    However, I found a deadlock in the scheduler when I tried
    similar aging tests with DVFS autonomous mode where CPU
    clock speed changed based on cpu load. In this case, call
    sequences were as follows;

    cpu1: sched_unlock()->sched_mergepending()->sched_addreadytorun()->up_cpu_pause()
    cpu0: sched_lock()->sched_mergepending()

    To avoid this deadlock, I added sched_tasklist_unlock() when calling
    up_cpu_pause() and sched_addreadytorun(). Also, added
    sched_tasklist_lock() after the call.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* libc: Add critical section in lib_filesem.c for SMP

    To set my...

net/tcp:  In tcp_input(), add the logic to detect, decode, and respond to TCP Keep-Alive packets.  In tcp_timer(), fix some problems with the way that TCP Keep-Alive probes were being formatted.

    net/tcp:  Add logic to send probes when SO_KEEPALIVE is enabled.
    net/tcp:  TCP socket should not have to be connected to configure KeepAlive.
    net/: Add a separate configuration to enable/disable KEEPALIVE socket options.
    net/tcp: Arguments to TCP keep-alive timing functions probably should be struct timeval as are the times for other time-related socket options.
    net/tcp:  Fix a backward conditional
    net/tcp:  Add some more checks and debug output to TCP-protocol socket options.
    net/tcp:  Cosmetic changes to some alignment.
    net/:  Adds socket options needed to manage TCP-keepalive and TCP state machine logic to detect if that the remote peer is alive.  Still missing the timer poll logic to send the keep-alive probes and the state machine logic to respond to probes.

net/tcp: tcp_send_[un]buffered.c:  Check routing table in psock_send_addrchck().  Previously only ARP table was considered when determining if the data will actually be sent.

Added ECANCELED condition to DEBUGASSERT-s checking sem_wait result

Approved-by: Gregory Nutt <gnutt@nuttx.org>

Fixed misprint in ipv4_forward.c

Approved-by: Gregory Nutt <gnutt@nuttx.org>

net/tcp:  Fixed bad return value handling in psock_tcp_send().  send() expects psock_tcp_send() to return a negated errno value, not -1 with the errno set (GN: I added same change for tcp_send_buffered.c which has the same issue as tcp_send_unbuffered.c)

    binfmt/, configs/, grahics/, libc/, mm/, net/, sched/:  OS references to the errno variable should always use the set_errno(), get_errno() macros
    arch/arm/src/stm32 and stm32f7:  Architecture-specific code is not permitted to modify the errno variable.  drivers/ and libc/:  OS references to the errno variable should always use the set_errno(), get_errno() macros

net/udp:  Fix memory leak with UDP + write buffer is closed.  Also update TODO and comments and refresh a configuration.

net/udp:  Resolves final design issues with UDP write buffering.  100% code complete but also 100% untested.

Squashed commit of the following:

    net/udp:  Address most of the issues with UDP write buffering.  There is a remaining issue with handling one network going down in a multi-network environment.  None of this has been test but it is certainly ready for test.  Hence, the feature is marked EXPERIMENTAL.
    net/udp:  Some baby steps toward a corrected write buffering design.
    net/udp:  Remove pesky write buffer macros.
    Eliminate trailing space at the end of lines.
    net/udp:  A little more UDP write buffering logic.  Still at least on big gaping hole in the design.
    net/udp:  Undefined CONFIG_NET_SENDTO_TIMEOUT.
    net/udp:  Crude, naive port of the TCP write buffering logic into UDP.  This commit is certainly non-functional and is simply a starting point for the implementatin of UDP write buffering.
    net/udp:  Rename udp/udp_psock_sendto.c udp/udp_psock_sendto_unbuffered.c.

net/udp:  Remove some conditional logic that was true if there is only a single network device, but not true in the multi-device context.

net/tcp:  Write buffering logic should not wait for a free buffer if the socket was opened non-blocking.  Also, rename the TCP write buffering macros from WRB_* to TCPWB_* to make room in the namespace for write buffering with other protocols.

net/udp:  Fix memory leak with UDP + write buffer is closed.  Also update TODO and comments and refresh a configuration.

net/udp:  Resolves final design issues with UDP write buffering.  100% code complete but also 100% untested.

Squashed commit of the following:

    net/udp:  Address most of the issues with UDP write buffering.  There is a remaining issue with handling one network going down in a multi-network environment.  None of this has been test but it is certainly ready for test.  Hence, the feature is marked EXPERIMENTAL.
    net/udp:  Some baby steps toward a corrected write buffering design.
    net/udp:  Remove pesky write buffer macros.
    Eliminate trailing space at the end of lines.
    net/udp:  A little more UDP write buffering logic.  Still at least on big gaping hole in the design.
    net/udp:  Undefined CONFIG_NET_SENDTO_TIMEOUT.
    net/udp:  Crude, naive port of the TCP write buffering logic into UDP.  This commit is certainly non-functional and is simply a starting point for the implementatin of UDP write buffering.
    net/udp:  Rename udp/udp_psock_sendto.c udp/udp_psock_sendto_unbuffered.c.

net/udp:  Remove some conditional logic that was true if there is only a single network device, but not true in the multi-device context.

net/tcp:  Write buffering logic should not wait for a free buffer if the socket was opened non-blocking.  Also, rename the TCP write buffering macros from WRB_* to TCPWB_* to make room in the namespace for write buffering with other protocols.