Add PID to UCred on remaining platforms

[Darksonn]

Implement pid on windows for UCred. See #3085 and #2633.

What other platforms are missing?

cc @de-vri-es, @Kloenk

[Kloenk]

IIRC I left open nearly all BSDs (only MacOS) and Solaris.

EDIT: Solaris got already implemented by somebody else

[Darksonn]

The currently supported platforms are listed here:

tokio/tokio/src/net/unix/ucred.rs

Lines 25 to 28 in 90c2a51

	/// Gets PID (process ID) of the process.
	///
	/// This is only implemented under Linux, Android, iOS, macOS, Solaris and
	/// Illumos. On other plaforms this will always return `None`.

[de-vri-es]

Windows doesn't have UCred at all right? Makes sense since Windows doesn't have Unix sockets. Although maybe there's something similar for pipes on Windows? I'm really not familiar with Windows.

OpenBSD and NetBSD do support PID though.

NetBSD supports it with the LOCAL_PEEREID socket option:
https://man.netbsd.org/NetBSD-9.0/unix.4

OpenBSD supports it with the SO_PEERCRED socket option:
https://man.openbsd.org/getsockopt.2

FreeBSD doesn't support it as far as I can tell. The struct returned with the LOCAL_PEERCRED option doesn't include a PID:
https://www.freebsd.org/cgi/man.cgi?query=unix&apropos=0&sektion=0&manpath=FreeBSD+12.2-RELEASE+and+Ports&arch=default&format=html

DragonFly BSD also doesn't seem to support it. They seem to have the same system as FreeBSD:
https://leaf.dragonflybsd.org/cgi/web-man?command=unix&section=ANY