Simplify handling of '.' and '-' after unbracketed named references.

Simplify handling of '.' and '-' after unbracketed named references.

* doc/bison.texinfo (Mid-Rule Actions): Mention that periods and
dashes make symbol names less convenient for named references.
* src/scan-code.l:
(handle_action_dollar): New arg textlen.  All callers changed.
(handle_action_at): Likewise.  Also, args are pointers to const.
(ref_tail_fields): Remove; no longer used.
(letter): Now includes '-' and '.', since this is for Bison
identifiers.
(id): Now the simpler traditional defn, since letters now include
'-' and '.'.
(c_letter, c_id): New defns.
(ref): Use c_id for unbracketed IDs.
(<SC_RULE_ACTION>): Simplify, now that the distinction between
Bison and unbracketed IDs are now in the regular expressions.
(VARIANT_BAD_BRACKETING): Remove.
(VARIANT_NOT_VISIBLE_FROM_MIDRULE): Renumber.
(find_prefix_end): Remove, replacing with ....
(identifier_matches): New function.
(variant_add): Use it.  Omit EXPLICIT_BRACKETING arg; no longer
needed.  CP arg is pointer to constant.  All callers changed.
(show_sub_messages): Remove args CP, EXPLICIT_BRACKETING, DOLLAR_OR_AT.
New arg TEXT.  All callers changed.  Do not worry about showing
trailing context.
(parse_ref): Args CP, RULE, TEXT are now pointers to const.  New
arg TEXTLEN.  Remove arg DOLLAR_OR_AT.  All callers changed.
Simplify code now that the regular expressions capture the
restrictions.
* src/scan-gram.l (letter, id): Adjust to match scan-code.l.
* src/symlist.c (symbol_list_null): Arg is now pointer to const.
* src/symlist.h: Likewise.
* tests/named-refs.at (Misleading references): These are now caught
by the C compiler, not by Bison; that's good enough.  Adjust test
to reflect this.
(Many kinds of errors, Unresolved references): Adjust expected
diagnostics to match new behavior.  The same errors are caught,
though the diagnostics are not quite as fancy.
($ or @ followed by . or -): Likewise.  Also, Make the grammar
unambiguous, so that diagnostics are not complicated by ambiguity
warnings.