wasip3: implement fcntl

[rvolosatovs]

Implement fcntl

Eventually, we should unify the UDP implementations (#728 (review)). If most other socket-related PRs are merged, I might tackle this next week Monday, otherwise it seems that doing so now would introduce quite a bit of churn.

This is blocked on #725

Unfortunately, due to the way sockets-nonblocking-udp is set up we cannot implement support for UDP just in socket and nowhere else, so bring along some fairly trivial UDP logic from wasip2 and introduce an abort() in poll for now.

wasi-libc/test/src/sockets-nonblocking-udp.c

Lines 37 to 38 in d974bce

	TEST(bind(server_socket_fd, (struct sockaddr *)&server_address,
	sizeof(server_address)) != -1);

[alexcrichton]

Upon reflection about #725, I actually think it'd be best to go in a different direction. WDYT about merging wasip{2,3}_tcp.c and tcp_sockopt.c all in one file? Similar for wasip{2,3}_udp.c and having that all in one file.

I'd like to by-default share things and we can split helpers out if necessary, but I think there's going to be a lot more shared code than not in the structure of these files.

[alexcrichton]

As a heads up if you're out of time on this @rvolosatovs I've done some work in #750 to resolve merge conflicts and address some thoughts of mine