ptserver: convert the ptserver to opr softsig

ptserver: convert the ptserver to opr softsig

Convert the ptserver from regular signal handling to the opr soft
signal handling when built with pthreads. This makes it safe to call
pthread functions within signal handlers.

Change-Id: I43d345517c75e275d6896154a979a908181a1f39
Reviewed-on: https://gerrit.openafs.org/11997
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>