fix unintended observability of SIGABRT disposition reset via sigaction

fix unintended observability of SIGABRT disposition reset via sigaction

the existing abort locking logic in sigaction only accounted for
attempts to change the disposition, not attempts to observe the change
made by abort.

unfortunately the change is still observable in at least one other
place: inheritance of signal dispositions across exec and posix_spawn.
fixing these is a separate task and it's not even clear whether a
complete fix is possible.