aclocal: more granularity in acdir overriding

aclocal: more granularity in acdir overriding

Before this change, using the `--acdir' option caused aclocal to
redefine both the directory of automake-provided m4 macros and the
directory of third-party system-wide m4 macros.  With this change,
we deprecate the `--acdir' aclocal option, and introduce two new
options `--automake-acdir' and `--system-acdir', to allow for more
granularity.

* aclocal.in (@automake_includes, @system_includes,
@user_includes): Fix and extend comments.
(usage): Update.
(handle_acdir_option): New function.
(parse_arguments): Recognize new options `--system-acdir' and
`automake-acdir', and handle `--acdir' using the new function
above.  Simplify logic by assuming that the directory of
third-party system-wide m4 files always exists.
* tests/aclocal.in: Update to use the new options, instead of
the deprecated. `--acdir'.
* m4/dirlist: Move ...
* m4/acdir/dirlist: ... here.
* m4/Makefile.am (EXTRA_DIST): Update.
(m4datadir): Rename ...
(automake_acdir): ... to this.  Accordingly, ...
(dist_m4data_DATA): ... rename this ...
(dist_automake_ac_DATA): ... to this.
(system_acdir): New, directory.
(dist_system_ac_DATA): New, defined to an empty value; this will
ensure that the $(system_acdir) directory will be created by
"make install".
* tests/aclocal.test: Remove check about the `--print-ac-dir'
option of aclocal, it has been moved into ...
* tests/aclocal-print-acdir.test: ... this new test, and quite
extended.
* tests/aclocal-acdir.test: New test.
* tests/Makefile.am (TESTS): Add the new tests.
* NEWS, bootstrap: Update.
* doc/automake.texi (aclocal Options, Macro Search Path): Update.