datamodel: management: reverting to absolute path for unix-socket

After a little research, I decided that the easiest way to fix #926 (closed) is to go back to absolute path only for /management/unix-socket and disable relative path.

To use relative paths reliably would require more changes to the configuration modeling utilities, and I don't think that's worth the effort.

Added little warning to if /management/unix-socket is out of /rundir.