variables: parse.error

Implement, document, and test the replacement of %error-verbose
by %define parse.error "verbose".

	* data/bison.m4 (b4_error_verbose_if): Reimplement to track the
	values of the parse.error variable.
	Make "simple" its default value.
	Check the valid values.
	* src/parse-gram.y: Use %define parse.error.
	(PERCENT_ERROR_VERBOSE): New token.
	Support it.
	* src/scan-gram.l: Support %error-verbose.

	* doc/bison.texinfo (Decl Summary): Replace the documentation of
	%define error-verbose by that of %define parse.error.
	* NEWS: Document it.

	* tests/actions.at, tests/calc.at: Use parse.error instead of
	%error-verbose.

src/scan-gram.l

@@ -194,7 +194,7 @@ splice	 (\\[ \f\t\v]*\n)*
   "%defines"                        return PERCENT_DEFINES;
   "%destructor"                     return PERCENT_DESTRUCTOR;
   "%dprec"                          return PERCENT_DPREC;
-  "%error"[-_]"verbose"             RETURN_PERCENT_FLAG("error-verbose");
+  "%error"[-_]"verbose"             return PERCENT_ERROR_VERBOSE;
   "%expect"                         return PERCENT_EXPECT;
   "%expect"[-_]"rr"                 return PERCENT_EXPECT_RR;
   "%file-prefix"                    return PERCENT_FILE_PREFIX;