Extend the front-end API for %define variables to more completely

Extend the front-end API for %define variables to more completely
mirror the back-end.  This will be useful in the future.
* data/bison.m4 (b4_percent_define_get, b4_percent_define_ifdef):
Update comments to mention the new front-end counterparts of these
macros.
* src/muscle_tab.c (MUSCLE_COMMON_DECODE): New macro with common code
for muscle_string_decode and muscle_location_decode.
(muscle_string_decode): New static function.
(muscle_location_decode): Use MUSCLE_COMMON_DECODE.
(muscle_percent_define_get, muscle_percent_define_ifdef): New
functions.
(muscle_percent_define_flag_if): Use muscle_percent_define_ifdef and
muscle_percent_define_get to mimic the b4_percent_define_flag_if
implementation more closely.
(muscle_percent_define_invalid_value): New function.
* src/muscle_tab.h (muscle_percent_define_get,
muscle_percent_define_ifdef, muscle_percent_define_invalid_value):
Prototype.