Replace users of "int fd" with "evutil_socket_t fd" in portable code

Replace users of "int fd" with "evutil_socket_t fd" in portable code

Remeber, win32 has a socket type that's actually a handle, so if
there's a chance that code is run on win32, we can't use "int" as the
socket type.

This isn't a blind search-and-replace: sometimes an fd is really in
fact for a file, and not a socket at all.