Add 'testsuite -C dir'.
* lib/autotest/general.m4 (_AT_ARG_OPTION): Move missing argument
detection...
(AT_INIT) <PARSE_ARGS_END>: ...here, since -k always takes
argument.
<DEFAULTS>: Delay computation of variables based on $at_dir...
<PREPARE_TESTS>: ...to here, since -C can change $at_dir.
<TESTS>: Re-invoke via absolute name, since -C may be in effect.
<PARSE_ARGS>: Parse new option.
<HELP_TUNING>: Document it.
* tests/autotest.at (Choosing where testsuite is run): New test
for this feature.
(Keywords and ranges): Add test for missing -k argument.
* NEWS: Document this.
* doc/autoconf.texi (testsuite Invocation): Likewise.
Signed-off-by: Eric Blake <ebb9@byu.net>