storm: meters from 24 up, plainer parser, documentation from the review

Storm control used meters 0 to 39, the same meters the ACL hands out,
so a storm limit and an acl meter could overwrite each other. It now
uses 24 + port * 4 + type, meters 24 to 63.

parse_storm() is no longer reentrant and uses plain locals, looks the
type up in a while loop and returns straight from the one-line cases.
storm_show() uses plain locals too.

The documentation says how IGMP decides between mcast and umcast, how
to get from a meter index to its limit, where the IFG bit of a meter
is, and what an ingress bandwidth limit on the same port does to the
storm limit.

httpd/page_impl.c

@@ -843,12 +843,12 @@ void send_storm(void)
 			char_to_html(reg_bit_test(RTL837X_STORM_CTRL + (t << 2), i) ? '1' : '0');
 		slen += strtox(outbuf + slen, "\",\"pps\":\"");
 		for (t = 0; t < STORM_TYPES; t++) {
-			idx = (i << 2) | t;
+			idx = STORM_METER(i, t);
 			char_to_html(reg_bit_test(RTL837X_METER_MODE + ((idx >> 5) << 2), idx & 0x1f) ? '1' : '0');
 		}
 		slen += strtox(outbuf + slen, "\",\"rate\":\"");
 		for (t = 0; t < STORM_TYPES; t++) {
-			reg_read_m(RTL837X_METER_RATE + ((((i << 2) | t)) << 2));
+			reg_read_m(RTL837X_METER_RATE + (STORM_METER(i, t) << 2));
 			byte_to_html(sfr_data[1]);
 			byte_to_html(sfr_data[2]);
 			byte_to_html(sfr_data[3]);