Change daemon() call to xdaemon() to avoid confusion with libc versions.

The glibc version does not double-fork like Slurm's version.
Rename to xdaemon to avoid confusion and ensure no one tries to
remove it again without further investigation
(see commits 6f45a2bf/954288ad).

While here, simplify it as the arguements are unchanged in the three
calling locations.

Remove incorrect documentation as well - our xdaemon() call does
not close everything.