Added option -D which puts pazpar2 in background. This improves the

Added option -D which puts pazpar2 in background. This improves the
startup process when pazpar2 is used in start scripts such as Debian's
start-stop-daemon. Without -D, pazpar2 would have to be started using
-b for start-stop-daemon. Refer to man page start-stop-daemon(8).
This change also makes pazpar2 change uid before the daemon is running.
We just ensure that HTTP binding and YAZ log files, PID files are dealt with
before any setuid takes place.. Pazpar2 makes log entry with version
info. The daemon mode refuses to start if -l is not given. This is to ensure
that logfile output is not dumped to /dev/null by accident.