aclocal: add --aclocal-path option to override ACLOCAL_PATH envvar.

aclocal: add --aclocal-path option to override ACLOCAL_PATH envvar.

From https://lists.gnu.org/archive/html/automake-patches/2022-01/msg00029.html
(plus wording tweaks from Karl; and thanks to Bogdan for research).

* bin/aclocal.in (usage): relabel output of m4 search path
components to be somewhat more correct (Karl), although it's still
a mess.
(parse_arguments): add new option --aclocal-path=s.
(parse_ACLOCAL_PATH): use $aclocal_path instead of hardwiring envvar.
* doc/automake.texi (aclocal Options): document it.
(dirlist): new anchor so we can reference it.
(Modifying the Macro Search Path: @file{ACLOCAL_PATH}): mention
the option.
(Possible future incompatibility): tone down; it might never happen.
* t/aclocal-path-precedence.sh: test the option.
* NEWS: mention the new option.