wait: implement waitid()

waitid() has a number of additional features that waitpid() is missing:

- WNOWAIT is only accepted for waitid() on Linux (and possibly other
  platforms)
- Support for waiting on PID file descriptors on Linux

For now support is added for all platforms with waitid() that have proper
siginfo_t support in libc. NetBSD support is currently a work in progress
[1].

Tests for the signal/exit code are currently skipped on MIPS platforms due
to bugs in qemu-user's translation of siginfo_t (fixed in [2] and [3]; the
second fix is not in a released qemu version yet).

[1] https://github.com/rust-lang/libc/pull/2476
[2] https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg04810.html
[3] https://lists.nongnu.org/archive/html/qemu-devel/2021-10/msg05433.html

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