feat(cat): make the DTR/RTS lowering a setting

Both defaults break someone. 0.22.7 lowered the lines on Yaesu and
Kenwood and silenced a TS-990 whose interface needs RTS raised to
transmit; 0.22.8 stopped lowering them and an FT8 station reported its
output power wandering, cured by closing OpsLog — Windows raises both on
open and the interface reads one as PTT.

So it is now a checkbox on those two backends, off by default: that is
how they behaved before any of this. Xiegu keeps lowering them
unconditionally (its own report, its own explicit keying setting), and
the CI-V backend keeps the behaviour it has always had.

internal/cat/kenwood.go

@@ -88,6 +88,26 @@ type Kenwood struct {
 	// Kept only to put it in the error message: "not answering" and "answering
 	// something unreadable" are different faults with different fixes.
 	heard string
+
+	// lowerLines deasserts DTR and RTS after opening the port.
+	//
+	// Neither default is safe for everyone, which is why this is a setting and
+	// not a decision. Windows raises both lines on open: an interface that reads
+	// them as PTT then keys the rig for as long as OpsLog is running — reported
+	// as FT8 output power wandering, and gone the moment OpsLog was closed.
+	// Lowering them instead silences the radios whose USB-serial interface needs
+	// RTS asserted to transmit at all — a TS-990 that opened cleanly and answered
+	// nothing. Off by default: that is how this backend behaved for its whole
+	// life before the question came up.
+	lowerLines bool
+}
+
+// SetLowerLines chooses whether DTR and RTS are deasserted on connect. Set
+// before Connect.
+func (k *Kenwood) SetLowerLines(v bool) {
+	k.mu.Lock()
+	k.lowerLines = v
+	k.mu.Unlock()
 }
 
 // where names the link for a message, so an error does not read "COM @ 0 baud"
@@ -572,18 +592,12 @@ func (k *Kenwood) openPort() (serial.Port, error) {
 	if err != nil {
 		return nil, err
 	}
-	// The modem lines are LEFT ALONE.
-	//
-	// They were briefly deasserted here, to stop an interface that reads them as
-	// PTT from keying the rig on connect. That silenced radios instead: a TS-990
-	// on COM3 opened fine and answered nothing, because a great many USB-serial
-	// interfaces will not transmit with RTS low — hardware flow control, or an
-	// output stage the line enables. "Opened but the rig sent nothing" was this,
-	// and it arrived as "CAT stopped working after the update".
-	//
-	// The fault that change was written for was a Xiegu G90 behind a DE-19, and
-	// that backend now has an explicit setting for which line keys it. A rig
-	// whose PTT is a CAT command has no business touching DTR or RTS at all.
+	// The modem lines are only touched when the operator asks for it. See
+	// lowerLines: both defaults break somebody's station.
+	if k.lowerLines {
+		_ = p.SetDTR(false)
+		_ = p.SetRTS(false)
+	}
 	return p, nil
 }