Provide AC_VERSION, not m4_AUTOCONF_VERSION.

Provide AC_VERSION, not m4_AUTOCONF_VERSION.
* doc/autoconf.texi (Text processing Macros): Remove mention of
m4_AUTOCONF_VERSION, and leave m4_PACKAGE_VERSION undocumented
once again.
(Notices): Move AC_PREREQ...
(Versioning): ...to this new section, alongside the new AC_VERSION
alias for the undocumented m4_PACKAGE_VERSION.
* lib/m4sugar/m4sugar.m4 (m4_AUTOCONF_VERSION): Revert change.
* lib/autoconf/general.m4 (AC_VERSION): New macro.
* NEWS: Update to match this rename.
* tests/m4sugar.at (m4@&t@_version_compare): Remove tests of
m4_PACKAGE_VERSION.
* tests/tools.at (autoconf: AC_VERSION): New test.
Suggested by Paolo Bonzini and Benoit Sigoure.