debug(icom): log the raw packets seen during a control handshake

A rig that answers something unrecognised (a newer model, another firmware)
and a rig that answers nothing are different faults; a silent timeout hides
which. Capped at a dozen packets so a working handshake cannot flood the log.
Prompted by the first IC-7760 network attempt — which turned out to be aimed
at the console's IP, where nothing listens; the remote server lives on the RF
deck.

internal/cat/icomnet.go

@@ -712,6 +712,11 @@ func icnHandshake(c *net.UDPConn, myID uint32, cancel <-chan struct{}) (uint32,
 		}
 		typ := icnLE.Uint16(p[4:])
 		sentid := icnLE.Uint32(p[8:])
+		// Every packet the rig sends during the handshake, verbatim. A radio that
+		// answers SOMETHING unrecognised (a newer model, a different firmware) and
+		// a radio that answers nothing are different faults, and a silent timeout
+		// hides which one this is.
+		icnHandshakeProbe(p)
 		switch typ {
 		case 0x04: // iAmHere
 			remoteID = sentid
@@ -903,3 +908,19 @@ func icnPasscode(s string) []byte {
 	}
 	return out
 }
+
+// icnHandshakeProbe logs the first packets seen during a control handshake —
+// capped hard, because a working handshake would log for ever.
+var icnProbeCount int
+
+func icnHandshakeProbe(p []byte) {
+	if icnProbeCount >= 12 {
+		return
+	}
+	icnProbeCount++
+	n := len(p)
+	if n > 32 {
+		n = 32
+	}
+	debugLog.Printf("icom net: handshake rx %d bytes: % X", len(p), p[:n])
+}