Clean up xkb_sym_interpret a bit

Clean up xkb_sym_interpret a bit

First we split the LEVEL_ONE_ONLY bit off of the 'match' field, which
allows us to turn enum xkb_match_operation to a simple enum and remove
the need for MATCH_OP_MASK.

Next we rename 'act' to 'action', because we've settled on that
everywhere else.

Finally, SIMatchText is changed to not handle illegal values - it
shouldn't get any. This removes one usage of the GetBuffer hack.

Signed-off-by: Ran Benita <ran234@gmail.com>