weston: deprecate enable_tap in favour of enable-tap

This is to increase consistency in config option naming in weston.ini.
(Prefer hyphens over underscores to separate words.)

If enable_tap is present in weston.ini, an obnoxious error message is
logged with weston_log(). In terms of configuration, if enable-tap is
present, enable_tap is ignored.

Signed-off-by: Eric Toombs <3672-ewtoombs@users.noreply.gitlab.freedesktop.org>