output: make --skip to take arguments what will be skipped

Accidental typo in usage() caused realisation making skipping to fully
controllable is good idea.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

src/dhcpd-pools.h

@@ -267,6 +267,10 @@ struct conf_t {
 		header_limit:4,
 		number_limit:3,
 		skip_ok:1,
+		skip_warning:1,
+		skip_critical:1,
+		skip_minsize:1,
+		skip_suppressed:1,
 		color_mode:2;
 };