Allow subset of wait4(2) functionality in Capsicum mode

[trasz]

Summary

The usual way of handling process exit exit in capsicum(4) mode is by using process
descriptors (pdfork(2)) instead of the traditional fork(2)/wait4(2) API. But most apps
hadn't been converted this way, and many cannot because the wait is hidden behind
a library APIs that revolve around PID numbers and not descriptors; GLib's
g_spawn_check_wait_status(3) is one example.

Thus, provide backwards compatibility by allowing the wait(2) family of functions
in Capsicum mode, except for child processes created by pdfork(2).

One practical result is that with this patch, a capsicated irssi(1) is no longer
leaving zombies around.

[oshogbo]

I think this behavior should be documented in the man pages (capsicum/wait), as it isn't obvious.

[oshogbo]

Also, do you have any particular cases or example of application where you want to use it?

[brooks]

What about wait6?

Should there be a way for a program to disable this?

[trasz]

I agree this should be documented somewhere, but at the moment wait(2) doesn't mention Capsicum at all, and capsicum(4) doesn't mention wait(2). Perhaps a paragraph in pdfork(2), something along the lines of "processes created with pdfork cannot be waited for by a parent running in capsicum(4) mode"?

Regarding use case - sh(1) when running with D44373 in place. Also either bmake(1) or clang(1) in a similar circumstances.

As for wait6(2) - definitely; I'll update the patch. Should I also handle wait(2) similarly, or is that just for backward compatibility?

As for disabling - I'm not sure. Are there cases where we'd need to?

[trasz]

Fix panic which occured when the PID is specified explictly.
Also handle wait6(2). Add some documentation. Pacify a test.