Always remove, invalidate and free watches before closing watched sockets

Always remove, invalidate and free watches before closing watched sockets

This should mean we don't get invalid fds in the main loop.

The BSD (kqueue) and Windows code paths are untested, but follow the same
patterns as the tested Linux/generic Unix versions.

DBusTransportSocket was already OK (it called free_watches() before
_dbus_close_socket, and that did the remove, invalidate, unref dance).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=33336
Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>
Reviewed-by: Thiago Macieira <thiago@kde.org>