configure: Improve help behavior

Old:
There are two paths to show help and exit 1, one is with "-h" or
"--help", one is with invalid options.

New:
Show help and exit 0 for --help.
On invalid option, don't show the long help and bury the early "ERROR:"
line, just give a message pointing to --help.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
