Improve AT_BANNER handling.

Improve AT_BANNER handling.

* lib/autotest/general.m4 (BANNERS): New named diversion.
(TESTS_END): Diversion no longer used.
(AT_INIT) <at_func_banner, BANNERS>: Factor all banners into a
shell function, which prints only as needed, using an associative
array of banner text from a special diversion.
<PARSE_ARGS_END>: No longer need awk to find banners.
<TESTS>: Banners are no longer processed by main driver loop, so
we no longer need case statement.
(AT_BANNER): Rewrite to populate new diversion.
(AT_SETUP): Each test invokes its own banner.  No output is needed
to the TESTS diversion.
* doc/autoconf.texi (Writing Testsuites): Document slight
semantics change.
* tests/autotest.at (AT_BANNERS): Enhance test.
* NEWS: Document AT_BANNER.

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