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

configure

@@ -1087,7 +1087,10 @@ for opt do
   ;;
   --enable-quorum) quorum="yes"
   ;;
-  *) echo "ERROR: unknown option $opt"; show_help="yes"
+  *)
+      echo "ERROR: unknown option $opt"
+      echo "Try '$0 --help' for more information"
+      exit 1
   ;;
   esac
 done
@@ -1353,7 +1356,7 @@ Advanced options (experts only):
 
 NOTE: The object files are built at the place where configure is launched
 EOF
-exit 1
+exit 0
 fi
 
 # Now we have handled --enable-tcg-interpreter and know we're not just