Add m4_reverse, and improve m4_list_cmp.

Add m4_reverse, and improve m4_list_cmp.

* lib/m4sugar/m4sugar.m4 (m4_reverse): New macro.
(m4_list_cmp): Rewrite to give linear behavior with M4 1.6 on an
m4_reverse'd list.
* lib/m4sugar/foreach.m4 (m4_reverse): Add the M4 1.4.x
counterpart.
* tests/m4sugar.at (recursion): Test it.
* doc/autoconf.texi (Evaluation Macros) <m4_reverse>: Document
it.
(Text processing Macros) <m4_append>: Cross-reference to m4_set.
* NEWS: Mention new macro.

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