Server: add internal PID file support to crabd

This makes use of the crab.util.pid module to implement
more careful PID file checking in the crabd server
launch script.  It should solve the recent fault where
the temporary directory was not cleared at system boot
and another process was running with the previous
crabd process number.  PID file handling has been
removed from crabd-check, other than determining
the name of the file to use.