Add user and group daemon options

Add start_listener_unix for socket file support

Allow nobody user/group to be provided by compiler as defines (so RedHat RPM can give nobody as the group to use)
Remove socket file before creating it, and umask during creation to set permissions
Moved xalloc_srs and xrealloc_srs into libcommon as xalloc_unsigned and xrealloc_unsigned
Improve naming of start_listener functions

Formatting fixes
Improve readability and fully separate declaration and code in listener funcs
Fix typo
Merge pull request #6 from driskell/master

Allow user and group specified from cmdline; and add unix socket utility functions