HPUX: Do not sigwait on critical signals

HPUX: Do not sigwait on critical signals

On HPUX, it is possible for 'critical' signals such as SEGV, ABRT, etc
to be delivered to the softsig thread when we sigwait(). The current
code marks these as 'fatal' and just exit(0)s when they are received,
preventing us from getting cores in the case of a SEGV, ABRT, etc.

To work around this and keep behavior on other platforms the same, just
do not wait on 'critical' signals on HPUX in the softsig thread.

Reviewed-on: http://gerrit.openafs.org/693
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>