Convert the vlserver 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: Ic9bd841c4796bd64b603505541da7e767afda83e
Reviewed-on: https://gerrit.openafs.org/11996 Reviewed-by: Perry Ruiter <pruiter@sinenomine.net> Reviewed-by: Benjamin Kaduk <kaduk@mit.edu> Tested-by: BuildBot <buildbot@rampaginggeek.com>