Miscellaneous %define and %code cleanup.

Miscellaneous %define and %code cleanup.
* data/bison.m4 (b4_percent_define_flag_if): Correct comments on how
values are interpreted.
* doc/bison.texinfo (Decl Summary): Clean up and extend %define
documentation a little more.
* src/muscle_tab.c (MUSCLE_USER_NAME_CONVERT,
muscle_percent_define_insert, muscle_percent_code_grow): New
functions/macros.
* src/muscle_tab.h (muscle_percent_define_insert,
muscle_percent_code_grow): Prototype.
* src/parse-gram.y (prologue_declaration): Use
muscle_percent_define_insert and muscle_percent_code_grow when parsing
%define and %code directives.

Make it easy to share %define boolean variables between the front-end
and back-end.  Though not used yet, this will be useful in the future.
* data/bison.m4 (b4_check_user_names): Rewrite comments to talk about
Bison uses of names rather than just skeleton uses of names.
(b4_percent_define_get, b4_percent_define_ifdef): Rename
b4_percent_define_skeleton_variables(VARIABLE) to
b4_percent_define_bison_variables(VARIABLE).
(b4_percent_code_get, b4_percent_code_ifdef): Rename
b4_percent_code_skeleton_qualifiers(QUALIFIER) to
b4_percent_code_bison_qualifiers(QUALIFIER).
(b4_check_user_names_wrap): Update for renames.
* src/muscle_tab.c, src/muscle_tab.h (muscle_percent_define_flag_if,
muscle_percent_define_default): New functions mimicking
b4_percent_define_flag_if and b4_percent_define_default.

For %define variables, report locations for invalid values and
redefinitions.
* data/bison.m4 (b4_percent_define_flag_if): Read
b4_percent_define_loc(VARIABLE) to report the location of an invalid
value for VARIABLE.
(b4_percent_define_default): Save a special location in
b4_percent_define_loc(VARIABLE) in case the default value for VARIABLE
must later be reported as invalid.
* src/muscle_tab.c (muscle_location_grow, muscle_location_decode): New
functions.
(muscle_percent_define_insert): Record the location of VARIABLE in
muscle percent_define_loc(VARIABLE), and use it to report the previous
location for a redefinition.
(muscle_percent_define_flag_if): Update like b4_percent_define_flag_if.
(muscle_percent_define_default): Update like b4_percent_define_default.
(muscle_grow_user_name_list): Rename to...
(muscle_user_name_list_grow): ... this for consistency and use
muscle_location_grow.
* src/muscle_tab.h (muscle_location_grow): Prototype.
* tests/input.at (%define errors): Update expected output.
* tests/skeletons.at (%define boolean variables: invalid skeleton
defaults): New test case.