Reorganize sockopts code and add several more socket options (#855)

Reorganize the sockopts code:
 - Split the `sockopt` modules out of the syscalls.rs files into their own files
 - Instead of duplicating all the documentation links for each socket option function, add a module-level documentation comment with all the links and just have the function documentation link to that.
 - Reorganize the sockopt tests so that the socket and tcp options are tested on both ipv4 and ipv6 sockets.
 - Adjust the bindings to reduce the amount of `as _` casting needed.

And, implement several more sockopt functions:
 - Add `*_ifindex` variations of `IP_ADD_MEMBERSHIP` and `IP_DROP_MEMBERSHIP`
 - Implement `IP_DROP_SOURCE_MEMBERSHIP`, `TCP_USER_TIMEOUT`, `IP_TOS`, `IPV6_RECVTCLASS`, `IP_RECVTOS`, `IP_ADD_SOURCE_MEMBERSHIP`, and `SO_OOBINLINE`.

src/backend/libc/net/mod.rs

@@ -4,5 +4,7 @@ pub(crate) mod ext;
 pub(crate) mod msghdr;
 pub(crate) mod read_sockaddr;
 pub(crate) mod send_recv;
+#[cfg(not(any(target_os = "redox", target_os = "wasi")))]
+pub(crate) mod sockopt;
 pub(crate) mod syscalls;
 pub(crate) mod write_sockaddr;