* aclocal.in (usage, parse_arguments): New --dry-run and --diff

* aclocal.in (usage, parse_arguments): New --dry-run and --diff
options.
(install_file, write_aclocal): Honor --dry-run and --diff.
* doc/automake.texi (aclocal options): Document --dry-run and --diff.
(Serials): Mention --diff.
* lib/Automake/FileUtils.pm (handle_exec_errors): Accept an
$expected_exit_code argument.
(xsystem): Take a list of arguments, not a string.
* tests/acloca18.test: Use --dry-run and --diff.
Suggested by Paul Eggert.