use ctypes instead of pbs_python

freealloc was originally written to use pbs_python, however
pbs_python 4.3.7 and earlier only works with TORQUE 4.1.x and
earlier -- see https://oss.trac.surfsara.nl/pbs_python/ticket/34

To work around this, freealloc was using the client pieces of
TORQUE 4.1.5.1 with pbs_python, when running on a cluster that
was using TORQUE 4.2.3.  However, after the cluster was upgraded
to TORQUE 4.2.5, the TORQUE 4.1.5.1 client stopped working with
pbs-server 4.2.5, returning the error "parse_daemon_response
error 15004 Invalid request".

The problem was solved by getting rid of both pbs_python and
the TORQUE 4.1.5.1 client entirely.  Instead, freealloc is now
using using Python's ctypes package.  While this is more lines
of code (in the freealloc script) and considerably less flexible
than using pbs_python, freealloc only calls three TORQUE library
functions (pbs_default, pbs_connect, and pbs_statjob), and so
using ctypes turned out to be easier than trying to resolve the
pbs_python and TORQUE version/compatibility-related problems.