muscle: enforce definition syntax for keyword variables

* src/muscle-tab.c (muscle_percent_define_get_kind)
(muscle_percent_define_check_kind): New.
(muscle_percent_define_default): Variables with a default value are
of "keyword" kind.
(muscle_percent_define_flag_if, muscle_percent_define_check_values):
Check that the variable is of keyword kind.
* data/bison.m4: Likewise, but in M4.  That is to say...
(b4_percent_define_default): Define the kind when the variable is undefined.
(b4_percent_define_check_kind): Use a better error message.
(_b4_percent_define_check_values, _b4_percent_define_check_values):
Former "enum" variables should be defined using the keyword syntax.
* doc/bison.texi: Update.
A couple of fixes.
* tests/input.at (%define keyword variables): New.