Move the DNS server from apps/netutils/dnsclient to nuttx/libc/netdb.  Move netdb functions from libc/net to libc/netdb.  Fix up naming to reflect the repartitioning

PCA9555: Extend interface to include methods for reading more than one bit at a time. From Sébastien Lorquet.

Protected mode:  Redesign how the user space heap is accessed from the kernel code.  It used to call memory management functions in user space via function pointers in the userspace interface.  That is inefficient because the first thing that those memory management functions do is to trap back into the kernel to get the current PID.  Worse, that operation can be fatal is certain fragile situations such as when a task is exitting.

The solution is to remove all of the memory management function calls from the interface.  Instead, the interface exports the userspace heap structure and then kernel size implementations of those memory management functions will operate on the userspace heap structure.  This avoids the unnecessary system calls and, more importantly, failures do to freeing memory when a test exits.

More changes to LPC3450 pin configuration:  The hardware ETM pins will not toggle unless the SLEW FAST bit is set.  From  Alessandro Temil.

-headers in nuttx/ioexpander to define the "generic" ioexpander framework
-files in drivers/ioexpander for the implementation of the NXP PCA9555 framework
-bindings in drivers/Kconfig and drivers/Makefile

From Sebastien Lorquet

The libc versions of get/sethostname should just call the internal net/netdb versions in the flat build.  In the protected and kernel builds, sethostname is a system call but gethostname calls uname() to get the hostname

gethostname() should not be the system call, uname() should and gethostname should get the host name from uname

get/sethostname:  Add a critical section.  There is a microscopic possibily that the global name could change while being accessed by a thread

uname: version string must hold 40 character value; strings returned by uname must always be NUL terminated

SAMD21: Last fix to header files caused a compile problem because logic used a value which is no longer available for the SAMD21

Add support for a BOARDIOC_RESET boardctl() command.  Enable configuration options to support BOARDIOC_RESET for ARMv6-M and ARMv7-M