sockopt_impl hygiene

[Frost-54]

What does this PR do

Currently

nix/src/sys/socket/sockopt.rs

Lines 180 to 185 in fb79966

	macro_rules! sockopt_impl {
	($(#[$attr:meta])* $name:ident, GetOnly, $level:expr, $flag:path, bool) => {
	sockopt_impl!($(#[$attr])*
	$name, GetOnly, $level, $flag, bool, $crate::sys::socket::sockopt::GetBool);
	};

references other macros and libc defined in nix without $crate.
This will cause compile error when using it outside of nix.

This PR adds $crate:: to all usages of macros in sockopt_impl and reference libc using $crate::libc.

However, just adding $crate:: will trigger deny(macro_expanded_macro_exports_accessed_by_absolute_paths), because pub mod socket is defined in a feature! macro.

nix/src/sys/mod.rs

Lines 129 to 133 in fb79966

	feature! {
	#![feature = "socket"]
	#[allow(missing_docs)]
	pub mod socket;
	}

I just manually expand feature! to work around it.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API