Don't let Bison leak memory except when it complains.

* src/files.h (parser_file_name, spec_verbose_file, spec_graph_file):
(spec_defines_file, dir_prefix): Now char *, not const char *,
since they are freed.
* src/files.c: Likewise.
(all_but_ext, all_but_tab_ext, src_extension, header_extension):
Likewise.
(tr): Now operates in-place.  All uses changed.
(compute_exts_from_gf, compute_exts_from_src): Don't leak temporary
values.
(compute_file_name_parts, compute_output_file_names): Don't store
read-only data in variables that will be freed.
(compute_output_file_names): Free all_but_ext, all_but_tab_ext,
src_extension, and header_extension.
(output_file_names_free): New public function to free
spec_verbose_file, spec_graph_file, spec_defines_file,
parser_file_name, and dir_prefix.
* src/getargs.c (getargs): Don't store read-only data in variables that
will be freed.
* src/main.c (main): Invoke output_file_names_free, code_scanner_free
(which previously existed but was unused), and quotearg_free.
* src/muscle_tab.h (muscle_insert): value arg is now a `char const *'.
* src/muscle_tab.c: Likewise.
(muscle_entry): Make the value char const *,
and add a new storage member that is char * and can be freed.
(muscle_entry_free): New private function.
(muscle_init): Use it instead of free.
(muscle_insert, muscle_grow): Update and use new storage member.
(muscle_code_grow): Free the string passed to muscle_grow
since it's not needed anymore.
* src/parse-gram.y (%union): Make `chars' member a `char const *', and
add a new `char *code' member.
("{...}"): Declare semantic type as code.
* src/scan-code.h (translate_rule_action):
(translate_symbol_action, translate_code, translate_action): Return
`char const *' rather than `char *' since external code should not free
these strings.
* src/scan-code.l: Likewise.
* src/scan-gram.l (<SC_BRACED_CODE>): Use val->code for BRACED_CODE,
which is "{...}" in the parser.
* tests/Makefile.am (maintainer-check-valgrind): Set
VALGRIND_OPTS='--leak-check=full --show-reacheable=yes' before invoking
Valgrind.
* tests/calc.at (_AT_DATA_CALC_Y): fclose the FILE* so Valgrind doesn't
complain.
* tests/testsuite.at (AT_CHECK): Redefine so that running Bison and
expecting a non-zero exit status sets --leak-check=summary and
--show-reachable=no for Valgrind.  Bison unabashedly leaks memory in
this case, and we don't want to hear about it.