Add ability to safely run multiple instances of innobackupex on the same machine

For XtraDB Cluster and other less common use cases, make sure that innobackupex can be safely run in parallel with proper validation to prevent incorrect usage. One specific known issue is the possible collision of the suspend file if a unique tmpdir is not specified.

Tangential issue is the detection of stuck/crashed instances which may leave behind stale files that are incorrectly recognized and used by currently running instance or detection of inappropriately run parallel instances with conflicting options (ex: running two instances with exact same tmpdir and outputting to same location).

Added:
Perhaps the only real useful work here is to have innobackupex place PID/semaphore files in the tmpdir and backup location (if not streaming) during init and remove on exit and validate that these files do not exist on init. This would prevent someone from running parallel instances with same tmpdir and backupdir but still allow unique parallel runs.

Another option would be to expand on the tmpdir similar to backupdir w/ timestamped directory name.