Refactored code into the parser to allow the freeform boolean types used
in Option entries to be used in other configuration entries. This isn't
as powerful as allowing "No" to precede the option names, but it atleast
gives a common handling of "yes", "no", etc.

A type xf86TriState has been added to support an optional boolean. This
allows the boolean sense of the value to be kept while providing a means
to signal that it is unset.

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer at who-t.net>