apps/netutils/netlib:  Add utilities to convert to/from prefix lengths from/to 128-bit network masks.  Modify the NSH IPv6 ifconfig command to show the network mask in a more standard, human readable way.

ICMPv6:  Some initial bugfix from first tests of the ICMPv6 auto-configuration logic.  so far so good but a lot more testing is needed

Build-server sometimes fails 'make -j24' with:

 In file included from builtin_list.c:62:0:
 builtin_list.h:1:46: error: 'ts_engine_main' undeclared here (not in a function)
  { "ts_engine", SCHED_PRIORITY_DEFAULT, 2048, ts_engine_main },

or sometimes silently succeeds but generates builds that contain
a random subset of configured NuttX applications. There are two
root causes for this:

1) Recipes for building builtin_list.h and builtin_proto.h are not
   linearizable.
2) Nothing ensures that 'make context' is run first for apps/builtin

This patch addresses both issues.

Signed-off-by: Juha Niskanen <juha.niskanen@haltian.com>

drivers/ramdisk.c and include/nuttx/fs/ramdisk.h:  Add logic to dispose of the drvier and RAM buffer when the RAM disk has been unlinked and all open references to the RAM disk have been closed.  Add new parameters to romdisk() to specify what should be done with the RAM/ROM buffer -- Should it be freed or not?  Changed all calls to ramdisk() to use these new parameters.

Unix domain/FIFOs:  Fix a race condition between FIFO buffer operations and the opening and closing of FIFOs which necessary when the FIFOs are used to support Unix domain, datagram sockets.  The default policy is the deallocate FIFO buffering when the last client closes the pipe.  When when used for datagram communicatinos, packets left in the FIFO will be lost.  Some like UDP read-ahead is needed:  The buffered data in the FIFO needs to be retained until the reader gets a chance to re-open the FIFO.  Added an ioctl (PIPEIOC_POLICY) to control the buffer policy. Default (0) is the legacy behavior; Unix domain datagram logic sets the alternative policy so that the packet data persists after the FIFO is closed.

Unix domain:  With these changes I can build a system with no link layer enabled, only Unix domain sockets