vos: Use IOMGR_SoftSig for signals

vos: Use IOMGR_SoftSig for signals

When vsprocs tries to longjmp from the signal handler, use IOMGR_SoftSig
first, so sleeping processes are woken back up. Without this, IOMGR can
bail out since it finds no processes READY when we attempt to do
anything with RX.

Change-Id: I9089bc4d003786d4a2205cd97f722fbf85fd1aa8
Reviewed-on: http://gerrit.openafs.org/1558
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>