Fix m4_map, and add some more utility macros.

Fix m4_map, and add some more utility macros.

* lib/m4sugar/m4sugar.m4 (m4_apply, m4_count, m4_dquote_elt)
(m4_echo, m4_make_list): New documented macros.
(_m4_quote, _m4_shift2): New helper macros.
(m4_map): Change semantics to allow calling macro without
arguments.
(m4_map_sep): Likewise.  Also change semantics to quote separator,
to match m4_join and m4_append.
(m4_version_unletter): Fix use of m4_map.
* doc/autoconf.texi (Evaluation Macros): Document m4_apply,
m4_count, m4_dquote_elt, m4_echo, m4_make_list.
(Text processing Macros): Mention m4_dquote as a faster
alternative to joining with commas.
(Looping constructs): Document m4_map, m4_map_sep.
* NEWS: Mention new macros.

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