`std.posix` should probably not pretend that `sigaction` can actually be missing

[alexrp]

zig/lib/std/posix.zig

Lines 5660 to 5667 in 8267929

	/// Examine and change a signal action.
	pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) error{OperationNotSupported}!void {
	switch (errno(system.sigaction(sig, act, oact))) {
	.SUCCESS => return,
	.INVAL, .NOSYS => return error.OperationNotSupported,
	else => unreachable,
	}
	}

Why are we pretending that sigaction can be missing (NOSYS) on POSIX? This is an extremely fundamental function. I've worked on several projects that needed to run on bizarre and broken "POSIX-ish" systems, and I don't recall ever having to worry that sigaction of all things was missing.

Any objections to removing NOSYS from that prong?