Don't use m4_divert since it makes m4_divert_push and m4_divert_pop

Don't use m4_divert since it makes m4_divert_push and m4_divert_pop
unreliable -- especially when they're hidden inside another macro.
* data/bison.m4, data/c++-skel.m4, data/c++.m4, data/c-skel.m4,
data/c.m4: Remove m4_divert(-1).
* data/glr.c, data/glr.cc, data/lalr1.cc, data/location.cc,
data/push.c, data/yacc.c: Likewise, and replace m4_divert(0) with
m4_divert_push(0) and m4_divert_pop(0).
* data/output.c (output_skeleton): Don't add an m4_divert_push(0) and
an m4_wrap([m4_divert_pop(0)]) to the M4.  Diversion -1, which is
pushed and popped by m4sugar, should be first on the stack.

Provide warn, complain, and fatal function callbacks to the skeletons.
This provides more flexibility than m4_fatal, improves the error
message format, and captures messages for translation.  Discussed
starting at
<http://lists.gnu.org/archive/html/bison-patches/2006-12/msg00063.html>.
* data/bison.m4 (b4_error): New, invoked by...
(b4_warn, b4_complain, b4_fatal): ... these new macros to wrap the
skeleton scanner's new @warn(...@), @complain(...@), and @fatal(...@)
directives.  Because these M4 macros might be called when the current
diversion is -1 or 0, m4_divert_push and m4_divert_pop is used; thus
the previous removal of uses of m4_divert, which caused trouble.
(b4_check_percent_code_qualifiers): Use b4_complain instead of
m4_fatal to report unrecognized %code qualifiers.
* data/c++-skel.m4: Use b4_complain instead of m4_fatal to report C++
push parser requests.
* data/glr.c: Use b4_complain instead of m4_fatal to report
non-deterministic push parser requests.
Update @output usage to @output(...@) form.
* data/glr.cc, data/lalr1.cc: Use b4_fatal instead of m4_fatal to
report missing %defines.  Update @output usage to @output(...@) form.
* data/location.cc, data/push.c, data/yacc.c: Update @output usage to
@output(...@) form.
* src/main.c (main): Invoke skel_scanner_free.
* src/scan-skel.h (skel_scanner_free): Prototype new function.
* src/scan-skel.l (FLEX_NO_OBSTACK): Don't define; we now need the
obstack_for_string from flex-scanner.h.
(YY_DECL): Use to declare skel_lex static.
(decode_at_digraphs): Remove; now handled in the new
SC_AT_DIRECTIVE_ARG start condition.
(fail_for_at_directive_too_many_args, fail_for_invalid_at): New static
functions.
(at_directive_name, AT_DIRECTIVE_ARGC_MAX, at_directive_argc,
at_directive_argv): New static globals.
(INITIAL): Use fail_for_invalid_at.
Don't parse `@output file_name\n' or `@basename(...@)'.  Instead,
recognize the start of a generalized `@directive(...@)' form and
start...
(SC_AT_DIRECTIVE_ARG): ... this new start condition to parse the
directive args (using the new obstack_for_string), to decode the
contained @ diagraphs, and to perform the directive.  It recognizes
@basename(...@), @warn(...@), @complain(...@), @fatal(...@), and
@output(...@).
(SC_AT_DIRECTIVE_SKIP_WS): New start condition started by
SC_AT_DIRECTIVE_ARG to skip whitespace after the argument delimiter,
`@,'.
(scan_skel): Initialize obstack_for_string on the first call.
(skel_scanner_free): New function to free obstack_for_string.
* tests/input.at (Reject bad %code qualifiers): Update test output.