fix: serial CAT keyed the radio on connect; Xiegu can key on RTS/DTR

Reported on a Xiegu G90. With the blue programming cable — three wires, no
modem lines — CAT works perfectly. Behind a DE-19, which carries audio, CAT and
PTT, the radio went into transmit the moment OpsLog connected and stayed there.

Windows raises DTR and RTS when a serial port is opened, and the DE-19 reads
them as PTT; Xiegu's own documentation asks for both low. The CI-V backend has
dropped them since it was written, for the same reason on Icom rigs with USB
SEND mapped to a line. Xiegu, Yaesu and Kenwood did not. They do now.

The second half of the report: WSJT-X through OpsLog's rigctld decoded fine and
never transmitted. Nothing was broken in that chain — set_ptt reaches the
backend, which sends the CI-V PTT command, which a G90 ignores. That is why
Xiegu keys on a hardware line instead. The backend can now do that: Settings →
CAT → Xiegu → how the rig is keyed (CI-V / RTS / DTR).

Untested here — no G90 in reach. The operator who reported it offered to try.

changelog.json

@@ -1,4 +1,16 @@
 [
+  {
+    "version": "0.22.7",
+    "date": "",
+    "en": [
+      "Serial CAT no longer keys the radio just by connecting: DTR and RTS are dropped on Xiegu, Yaesu and Kenwood as they already were on Icom — a Xiegu G90 behind a DE-19 went straight into transmit and stayed there.",
+      "Xiegu: choose how the rig is keyed — CI-V command, RTS or DTR. A G90 does not transmit on the CI-V command, so this is also what lets WSJT-X transmit through the shared CAT link."
+    ],
+    "fr": [
+      "Le CAT série ne met plus la radio en émission par le seul fait de se connecter : DTR et RTS sont abaissés sur Xiegu, Yaesu et Kenwood comme ils l'étaient déjà sur Icom — un Xiegu G90 derrière un DE-19 partait en émission et y restait.",
+      "Xiegu : choix du passage en émission — commande CI-V, RTS ou DTR. Un G90 n'émet pas sur la commande CI-V, c'est donc aussi ce qui permet à WSJT-X d'émettre via le partage CAT."
+    ]
+  },
   {
     "version": "0.22.6",
     "date": "",