Add Checkpoint/Restart support for UNIX sockets

This is the latest version of my patch set enabling c/r for sockets.  I
chose to send only the UNIX patch and supporting changes to concentrate
on that as the INET patch will need more work.

This set introduces three c/r-specific helper routines (patches 1-3), a
more general sock_bind() helper to encapsulate the security hook performed
by sys_bind() (patch 4), and finally the C/R of UNIX sockets (patch 5).
There have been substantial changes to the UNIX patch, detailed accordingly.
Hopefully I have touched on all the previous comments.