SIGSYS annoyance

[Andy Lutomirski]

https://bugzilla.mozilla.org/show_bug.cgi?id=1176099

Should SIGSYS be delivered to the handler even if blocked?  What, if
anything, does POSIX say?  All I can find is in pthread_sigmask(3p):

If any of the SIGFPE, SIGILL, SIGSEGV, or SIGBUS signals are generated
while they are blocked, the result is undefined, unless the signal was
generated by the action of another process, or by one of the functions
kill(), pthread_kill(), raise(), or sigqueue().

It would be easy enough to change our behavior so that we deliver the
signal even if it's blocked or to at least add a flag so that users
can request that behavior.