Improve AC_DEFUN_ONCE semantics.

Improve AC_DEFUN_ONCE semantics.

* lib/m4sugar/m4sugar.m4 (m4_defun_once): Rewrite to be no-op,
rather than warning, on second use, and make sure first use never
occurs out of order.
* tests/m4sugar.at (m4@&t@_require: one-shot initialization):
Enhance test.
* tests/base.at (AC_REQUIRE & AC_DEFUN_ONCE: [Require, expand],
(AC_REQUIRE & AC_DEFUN_ONCE: [Expand, require]): Adjust tests.
* NEWS: Document this.
* doc/autoconf.texi (Macro Definitions) <AC_DEFUN>: Mention
AC_DEFUN_ONCE.
(Prerequisite Macros) <AC_REQUIRE>: Likewise.
(Expanded Before Required): Likewise.
(One-Shot Macros) <AC_DEFUN_ONCE>: Document new semantics.
Reported by Bruno Haible, with suggestion by Paolo Bonzini.

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