Support unbalanced () in AT_SETUP by adding two new quadrigraphs.
* bin/autom4te.in (handle_output): Substitute @{:@ and @:}@.
(handle_traces): Likewise.
* lib/m4sugar/m4sugar.m4 (m4_qlen): Account for new quadrigraphs.
* tests/autotest.at (AT_CHECK_AT_TITLE_CHAR): Add new tests.
* doc/autoconf.texi (Quadrigraphs): Document them.
(Evaluation Macros) <m4_expand>: Enhance documentation.
(Text processing Macros) <m4_text_box>: Document cases where
quadrigraphs can help for problemetic unbalanced parentheses.
(Pretty Help Strings) <AS_HELP_STRING>: Likewise.
(Writing Testsuites) <AT_SETUP>: Likewise.
(Limitations of Builtins) <case>: Consolidate text on unbalanced
parentheses, and add an example of creative comments.
* NEWS: Document the addition.
Reported by Joel E. Denny.
Signed-off-by: Eric Blake <ebb9@byu.net>