Add Darwin support

This MR allows Wayland to be available on Darwin systems (macOS, iOS, etc.).

• Changes
  • Enable epoll-shim for Darwin (https://github.com/jiixyj/epoll-shim/commit/36f34cbb9ff29a8966dede4ab34b626df510a2d1)
  • Compatibility changes related with socket and accept
  • Disable tests/os-wrappers-test.c on Darwin as no wrap_* support

Taken advantage of !290, !303, !254 but not cherry-picking because code has been changed.

Fixed #310