Monitor: --oneshot must imply --no-sharing

Moving spares is not the purpose of '--oneshot' and without
currently --oneshot doesn't work as the check against multiple
spare-sharers prevents it.

So make --oneshot imply --no-sharing.  This is sensible and allows
--oneshot to work again.

Reported-by: Jamie Heilman <jamie@audible.transient.net>
Resolves-Debian-Bug: 641886
Signed-off-by: NeilBrown <neilb@suse.de>

mdadm.c

@@ -841,6 +841,7 @@ int main(int argc, char *argv[])
 			continue;
 		case O(MONITOR,'1'): /* oneshot */
 			oneshot = 1;
+			spare_sharing = 0;
 			continue;
 		case O(MONITOR,'t'): /* test */
 			test = 1;