linuxulator: Accept 0 as signal in rt_sigqueueinfo

[rmoeller]

Summary

Linux allows 0 as a signal for rt_sigqueueinfo(2) the same as it does
for kill(2). This is used to check for process existence and
permissions.

Allow this value through as a special case in linux_rt_siqgueueinfo the
same way as was done for linux_kill.

Before this change, lio_listio(3) would fail with EINVAL under
Linuxulator. Now it is functional.

MFC after: 1 week

[rmoeller]

With this change I can run zdb (and other libzpool-based tooling) compiled for Linux under Linuxulator.

[rmoeller]

sys/compat/linux/linux_signal.c

544–558

Note the same pattern here.

566–583

And here.