Prefer signed types for indexes in skeletons

Prefer signed types for indexes in skeletons

* NEWS: Mention this.
* data/skeletons/c.m4 (b4_int_type):
Prefer char if it will do, and prefer signed types to unsigned if
either will do.
* data/skeletons/glr.c (yy_reduce_print): No need to
convert rule line to unsigned long.
(yyrecoverSyntaxError): Put action into an int to
avoid GCC warning of using a char subscript.
* data/skeletons/lalr1.cc (yy_lac_check_, yysyntax_error_):
Prefer ptrdiff_t to size_t.
* data/skeletons/yacc.c (b4_int_type):
Prefer signed types to unsigned if either will do.
* data/skeletons/yacc.c (b4_declare_parser_state_variables):
(YYSTACK_RELOCATE, YYCOPY, yy_lac_stack_realloc, yy_lac)
(yytnamerr, yysyntax_error, yyparse): Prefer ptrdiff_t to size_t.
(YYPTRDIFF_T, YYPTRDIFF_MAXIMUM): New macros.
(YYSIZE_T): Fix "! defined YYSIZE_T" typo.
(YYSIZE_MAXIMUM): Take the minimum of PTRDIFF_MAX and SIZE_MAX.
(YYSIZEOF): New macro.
(YYSTACK_GAP_MAXIMUM, YYSTACK_BYTES, YYSTACK_RELOCATE)
(yy_lac_stack_realloc, yyparse): Use it.
(YYCOPY, yy_lac_stack_realloc): Cast to YYSIZE_T to pacify GCC.
(yy_reduce_print): Use int instead of unsigned long when int
will do.
(yy_lac_stack_realloc): Prefer long to unsigned long when
either will do.
* tests/regression.at: Adjust to these changes.