l2: name the group a learned address sits behind

A device on an aggregate is learned against whichever member the frame came in
on, and the hash moves that around: the same address changes port every time
the flow mix changes. The table was telling the truth and there was no way to
read it, since nothing on the page or the console says those two ports are one
link.

port_lag_of() already exists and already knows the answer, so both views ask
it. The console prints LAG1 in place of the member, and /l2.json carries a
"lag" field beside the port, 0 when the port stands alone, which is the shape
/stp.json already uses.

Costs 151 bytes of BANK1.

httpd/page_impl.c

@@ -359,6 +359,7 @@ void send_l2(uint16_t idx)
 	while (1) {
 		entries_left--;
 		uint8_t port = 0;
+		uint8_t lag;
 		reg_read_m(RTL837x_TBL_DATA_0);
 		REG_WRITE(RTL837x_TBL_DATA_0, sfr_data[0], sfr_data[1] & 0xfc, sfr_data[2] | (TBL_LUTREAD_NEXT_L2UC << 6), sfr_data[3]);
 
@@ -402,6 +403,9 @@ void send_l2(uint16_t idx)
 
 			port |= (sfr_data[3] & 0x3) << 2;
 			itoa_html(port);
+			slen += strtox(outbuf + slen, ",\"lag\":");
+			lag = port_lag_of(port);
+			itoa_html(lag == PORT_LAG_NONE ? 0 : lag + 1);
 		}
 
 		// Index