Add multi-sched support to pbs_snapshot

Forum discussion: http://community.pbspro.org/t/enhancing-pbs-snapshot-for-multi-sched/870
Pull Request: https://github.com/PBSPro/pbspro/pull/616
Background:
- With the multi-sched feature (PP-337: Multiple schedulers servicing the PBS cluster) being checked in, pbs_snapshot will need to be enhanced to capture information from all of the schedulers correctly. Right now, it captures the following information about the default scheduler:
  - qmgr list sched
  - sched_priv
  - sched_logs
- With multisched in the picture, qmgr list sched should still work out of the box to capture info about all schedulers. But sched_priv and sched_logs of the different schedulers will need to be captured separately for each scheduler.
Design Proposal:
- The first level of snapshot directory will capture sched_priv and sched_logs from all the different schedulers.
  - The rationale is that PBS saves sched_priv and sched_logs at the first level of PBS_HOME, so this would be consistent with that.
- For the default scheduler, they will still be called "sched_priv" and "sched_logs".
- For all the other schedulers, they will be named as 'sched_priv_<sched_name>' & 'sched_logs_<sched_name>' where 'sched_name' is the name of the respective scheduler.