output: add --skip-ok option

Omit ranges and shared networks that do not exceed warning or critical
thresholds.

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

src/dhcpd-pools.h

@@ -238,6 +238,7 @@ struct configuration_t {
 		all_as_shared:1,
 		header_limit:3,
 		number_limit:3,
+		skip_ok:1,
 		color_mode:2;
 };