pbs_mom: added support for process oom-killer tunables in Linux

Two new mom config variables have been added: 
   'job_oom_score_adjust'
   'mom_oom_immunize'. 

Both settings are linux-specific therefore implemented only in linux/mom_mach.c

First one sets /proc/<pid>/oom_adj for every spawned task according to configuration
Tested with single task jobs and with mpiexec, pbsdsh multi-task jobs

Second one prevents pbs_mom from being incidentally hit by oom killer.
Due to the oom_score scoring algorithm in EL5 pbs_mom is usually first process to be hit by oom killer - this variable prevents that unnecessary act of violence.