Updated the interface of the release_resources method to take the PID (ID)
of the process releasing the resource. Updated the format of the resource
counter file to store a list of current resource users. If the provided PID
is not in the list, release request is ignored (warning, not an error). The
final intent of this extension is to simplify releasing resources in signal
handlers (PID known in advance, currently allocated resources may not be
known atomically, e.g., request prepared but not acquired yet). Updated the
tests, all pass. Have not updated other users (ibis-job-run).
Minor: unset PGID variable after use, better comment on 'timeout -k'.