procmgmt: Introduce spawnprocve_sig

Introduce spawnprocve_sig, a variant of spawnprocve that allows
a caller to spawn a process with a specific signal mask.
This is useful when we want to set a mask that is different
from the current one.  It needs to be done after the fork()
so that the current thread is not affected.

Change-Id: I900c85cb70d22756b78562618b0e853dcedf8235
Reviewed-on: http://gerrit.openafs.org/8749
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>