pbs_mom init scripts use the pbs_mom lock file as pid file

When an interactive job is running, multiple pbs_mom children are spawned. If the parent pbs_mom fails or is killed (eg during service restart), service start fails because the child pbs_moms are seen as active pbs_mom parent process.
Using the lock file that contains the pid to check the status fixes this problem. 

remark: i have no clue when ```status -p <pidfile>``` was introduced in the init functions (it's available in at least EL5.8 and EL6.4).