Disable unset/unused mid-rule value warnings by default, and recognize

Disable unset/unused mid-rule value warnings by default, and recognize
--warnings=midrule-values to enable them.  Discussed starting at
<http://lists.gnu.org/archive/html/help-bison/2006-10/msg00030.html>.
* NEWS (2.3a+): Mention.
* src/getargs.c, src/getargs.h (warnings_args, warnings_types, enum
warnings): Add entry for midrule-values subargument.
* src/reader.c (symbol_should_be_used): Don't return true just because
the value is a set/used mid-rule value unless
--warnings=midrule-values was specified.
* tests/input.at (Unused values, Unused values before symbol
declarations): Run tests with and without --warnings=midrule-values.

* src/reader.c (check_and_convert_grammar): Use symbol_list_free rather
than LIST_FREE directly.