No flock support on Illumos

[RalfJung]

Rust supports flock on a number of OSes, but Illumos is not on the list:

rust/library/std/src/sys/fs/unix.rs

Lines 1336 to 1356 in 2371802

	#[cfg(any(
	target_os = "freebsd",
	target_os = "fuchsia",
	target_os = "linux",
	target_os = "netbsd",
	target_os = "openbsd",
	target_os = "cygwin",
	target_vendor = "apple",
	))]
	pub fn try_lock(&self) -> Result<(), TryLockError> {
	let result = cvt(unsafe { libc::flock(self.as_raw_fd(), libc::LOCK_EX | libc::LOCK_NB) });
	if let Err(err) = result {
	if err.kind() == io::ErrorKind::WouldBlock {
	Err(TryLockError::WouldBlock)
	} else {
	Err(TryLockError::Error(err))
	}
	} else {
	Ok(())
	}
	}

This is despite the fact that Illumos does seem to support flock:

https://github.com/illumos/illumos-gate/blob/80040569a359c61120972d882d97428e80dcab90/usr/src/man/man3c/flock.3c

flock support for Illumos was present before #132977. That PR talks only about Solaris in its title but actually removed flock support for a wide range of Unixes -- supposedly because they don't support it, which was claimed without giving any sources and which was apparently incorrect at least for Illumos, Fuchsia, and OpenBSD.

Judging from libc, solaris is the only cfg(unix) OS that should be excluded here:

https://github.com/rust-lang/libc/blob/abf01706808828e82f4132113ecfeee4fe6c0d46/src/unix/mod.rs#L1636-L1642

Cc @cberned @tgross35 @jclulow @pfmooney

[jclulow]

FWIW, we have not historically supported it in illumos (we had a broken ancient BSD compatibility version that it was correct not to use) but yes we've had it since that commit you linked, with more details in the bug.