Socket overhaul (1)

 * Internally, libev maintains a list of watchers for each monitored fd.
   Add ev_io_closing(fd), does ev_feed_event() for each watcher affected.

 * Add coio_close()  [Public API]
   Wakes all fibers blocked in coio_wait() AND closes the fd.
   Prerequisite for #886 Non-blocking popen.

 * Lua socket switched to coio_close(); stop tracking
   waiters explicitly.

 * socket ::= { _gc_socket = <ffi:struct gc_socket> }, _gc_socket
   was previously known as socket. Gc_socket stores the fd in
   immutable field.

 * Overall socket module cleanup.