Unix: add IN6ADDR_ANY_INIT, IN6ADDR_LOOPBACK_INIT, in6addr_any, in6addr_loopback

[safinaskar]

This is not breaking change.

Note that this PR adds global variables (statics) in6addr_any and in6addr_loopback. But this is okay, because:

• We already have global variables in crate libc: https://docs.rs/libc/0.2.154/aarch64-apple-darwin/libc/index.html#statics
• ctest2 knows how to test global variables: https://docs.rs/ctest2/0.4.8/ctest2/struct.TestGenerator.html#method.skip_static

This PR closes #1950

Also I suggest merging this PR for ctest2: JohnTitor/ctest2#56 , it removes annoying warnings produced by ctest2 in presence of statics. And also fixes CI for ctest2. I recommend merging JohnTitor/ctest2#56 , but this is not required

[rustbot]

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[tgross35]

These are defined here https://github.com/bminor/glibc/blob/b22923abb046311ac9097a36b97b9b97342bac44/inet/netinet/in.h#L237-L240. We do also have a few statics around, e.g.

libc/src/solid/mod.rs

Lines 838 to 842 in 50b4720

	pub static mut optarg: *mut c_char;
	pub static mut opterr: c_int;
	pub static mut optind: c_int;
	pub static mut optopt: c_int;
	pub static mut optreset: c_int;

.

LGTM, thanks!