volser: convert the volume server to opr softsig

volser: convert the volume server to opr softsig

Convert the volume server 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: I25b9a9184c526f4ce9b6e2abb25ae9135cc97ec6
Reviewed-on: https://gerrit.openafs.org/11995
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>