Avoid some overhead from m4_defn and m4_popdef.

Avoid some overhead from m4_defn and m4_popdef.

* lib/m4sugar/m4sugar.m4 (m4_defn, m4_popdef, m4_undefine): Only
pass on first argument, since we are documented that way.
(m4_for, m4_append_uniq, m4_text_wrap): Optimize out defined-ness
check where it is safe to do so.
(m4_append): Likewise, and quote the separator.
(m4_text_box): Likewise, and avoid regex, also be robust to
expansion and quadrigraphs.

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