Enhance AS_HELP_STRING.

Enhance AS_HELP_STRING.

* lib/m4sugar/m4sugar.m4 (m4_text_wrap): Don't expand arguments,
and reduce number of expansions.
* lib/m4sugar/m4sh.m4 (AS_HELP_STRING): Rework to use m4_expand,
and to take indent and wrap column numbers.
* tests/m4sh.at (AS@&t@_HELP_STRING): Update the test.
* doc/autoconf.texi (Pretty Help Strings): Document details about
arguments.
(Text processing Macros): Minor tweaks.
* NEWS: Document this change.

Signed-off-by: Eric Blake <ebb9@byu.net>