connection: Move wl_interface_equal to util

Move the wl_interface_equal prototype to the top of wayland-private, where
it is not buried in the middle of map, connection and closure functions.

Move the implementation out of connection and into util. This is a utility
function, not specific to connections, and has call sites within connection,
wayland-client and wayland-server.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>