Preliminary support for `aclocal --install'.

Preliminary support for `aclocal --install'.
This still lacks #serial support.
* aclocal.in (reset_maps, install_file): New functions.
(write_aclocal): Copy files if --install.
(usage, parse_arguments): Recognize --install.
("MAIN"): Start aclocal again if some file were installed.
* tests/acloca10.test: Augment to test --install.
* tests/aclocal.in, tests/defs.in: Add support for
ACLOCAL_TESTSUITE_FLAGS, used by acloca10.test.
* doc/automake.texi (aclocal options, Local Macros): Document
--install.
(Future of aclocal): Adjust.