Only invoke the old signal handler if it's a real signal handler
and not SIG_IGN, SIG_DFL, SIG_HOLD, or SIG_ERR, using new static
function issighandler().
Avoids an intermittent race condition with a null pointer
dereference via (*SIG_DFL)().
Bug class reported by Joyu Liao from Juniper Networks.
Use SIG_ERR instead of NULL as the indicator that a signal handler
hasn't been changed, so that SIG_DFL (equivalent to NULL)
will be restored.
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