Improve m4_copy.

Improve m4_copy.

* lib/m4sugar/m4sugar.m4 (m4_copy): Add second implementation for
public use.
(_m4_copy): New macro, which preserves pushdef stacks.
(_m4_defun_pro_outer): Bypass it, for speed.
(m4_init): Bypass new implementation, since it breaks on m4_defn.
* bin/autoupdate.in (handle_autoconf_macros): Likewise.
* lib/autoconf/general.m4 (AC_PREREQ): Undefine before redefining,
now that m4_copy checks this.
* doc/autoconf.texi (Redefined M4 Macros) <m4_copy>: Document
this, as well as m4_rename.
* lib/autoconf/autoconf.m4 (m4_copy): Temporarily redefine when
renaming builtins.
* NEWS: Likewise.
* tests/m4sugar.at (m4@&t@_defn): Enhance test.

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