Fixed i2ctool write operation in `no restart` mode (flag `-n`).

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

It seems that I2C driver has changed a bit since i2ctool was written,
so now i2ctool sends repeated start even if `no restart` flag (-n)
was passed to it.

include/netutils/netlib.h:  Add definitions needed when only local Unix domain sockets are supported.

Fix parsing of empty strings when CONFIG_NSH_CMDPARMS is not defined.  Problem noted by Juha Niskanen.

nshlib:  df command no long uses the non-standard OS interface foreach_mountpoint().  No simply cats /proc/fs/blocks or /proc/fs/usage.

nshlib:  mount command no long uses the non-standard OS interface foreach_mountpoint().  No simply cats /proc/fs/mount when there are no arguments to the mount command.

This commit removes the ping6 command from NSH and replaces it with the apps/system/ping6 built-in command.  The NSH ping6 command had to be removed because it violated the portable POSIX OS interface.  The apps/system/ping6 command uses the sem-standard IPPROTO_ICMP6 socket interface.

Squashed commit of the following:

    apps/system ping and ping6:  Fix a backward test.
    apps/system/ping6: ping6 logic now builds without error.
    apps/system/ping and ping6:  Minor clean-up to configuration settings.
    apps/system/ping6:  Minor clean-up to be able to configure CONFIG_SYSTEM_PING6
    apps/nshlib:  Remove support for ping6 from NSH.
    apps/system/ping6:  Add IPPROTO_ICMPv6 ping6.  Initial commit is simply a clone of the IPPROTO_ICMP ping logic.
    Rename CONFIG_NET_ICMPv6_PING to CONFIG_NET_ICMPv6_SOCKET.

This commite eliminates the ping command from NSH and replaces it with a ping 'built-in' at apps/system/ping.  The original NSH version of ping violated the portable POSIX interface and, hence, had to be removed.  The new system/ping command uses the new IPPROTO_ICMP AF_INET datagram sockets to implement ping.

Squashed commit of the following:

    apps/system/ping:  Fix some timing issues.
    apps/system/ping:  A few timing related corrections.
    apps/nshlib:  Remove support for the NSH 'ping' command.  The implementation of that command violated the portable POSIX interface and has been replaced with a ping 'built-in' command at apps/system/ping.
    apps/system/ping:  Add larger payload.  Verify content of echoed payload.
    apps/system/ping:  Various fixes for a clean compile
    system/ping: Add new build structure for system ping command.

    apps/fsutils/mkfatfs:  New user-space fatfs appears to work fine.
    apps/fsutils/mkfatfs:  Move mkfatfs from the OS to here.  Not fully integrated on the intial commit.

examples/alarm: add options for reading alarm value and canceling it

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

graphics/traveler:  Restore most of what was removed in the previous commit.  That non-standard logic is currently needed by the simulation.  It should be replaced with use of a framebuffer driver.