PP-515: quit mom safely when multinode jobs are present

Objective:
To cope with multinode jobs if you need to restart pbs_mom. 

Interface 1:
Admin can send SIGQUIT to pbs_mom and the pbs_mom distinguishes two states:

• no multinode job present - pbs_mom quits immediately
• multinode job is present - pbs_mom waits until no multinode job is present then quits

Notice:
Admin can start the pbs_mom with the -p parameter again with no loss of jobs.