Refactor unix domain socket naming to a separate file instead.

Handle selection between normal & abstract sockets in a single place.

overlay_mdp.c tried to have both but this seems like a bug so I've removed the second socket (needs tests).
Issue #20: merge branch 'sockets' into 'development'

Daniel's improvements to the local file/abstract socket code, with many
improvements to bring it up to date.
Improve fdqueue.c debug logging
Issue #20: improve new socket function logging

__whence argument and function macros
Issue #20: fix bug in new esocket() function
Issue #20: improve socket set-up logging
Fix Rom's socket cleanup-on-start code

Fix buffer overflow bug: replace call to sprintf(3) with call to
FORM_SERVAL_INSTANCE_PATH()

Fix file descriptor leak bug: add missing closedir(3) call

Use lstat() instead of stat() to avoid warnings on broken sym links

Use WARNF_perror() where appropriate in preference to WARNF()

Remove redundant one-line comments