Allow to start server and audiod as daemon with no logfile.

Currently para_audiod and para_server won't start in the background
if no logfile is specified, so "-dL /dev/null" must be given to force
this. This is a bit tedious to type, so this commit makes "/dev/null"
the default.

To achive this, we can simply remove the gengetopt "dependon" statement
from daemon.m4. This works because if no logfile was given, log output
is written to stderr, which is redirected to /dev/null in case -d
was also given. We need to open /dev/null in read-write mode though,
but no other changes are required.