fix(rigctld): stop refusing two commands JTDX sends in Fake It

From a user's log, repeating throughout the session:

	rigctld: split off failed: cat: this radio's backend cannot set split
	rigctld: unimplemented command "X PKTUSB -1"

Both are answered with an error, and JTDX treats a rig-control error as fatal:
it abandoned a transmission 0.86 seconds into a 13.8-second frame.

"S 0" — split OFF. Fake It uses no split, and JTDX still sends this to be sure.
A backend that cannot SET split was refusing a request that was ALREADY
satisfied. It now succeeds when the rig is simplex, without touching the rig at
all. Arming still fails loudly: there the request is real and unmet, and a
client that believes it transmits up the band while the radio sits on the DX's
frequency is exactly what that refusal exists to prevent.

"X <mode>" — set_split_mode. Sent during ordinary setup, Fake It included,
where there is no second VFO to give a mode to. Accepted now: applied when a
split is in force, and a plain success otherwise, because the transmit VFO
already has that mode — it is the same VFO. get_split_mode answers to match.

Both regressions fail without the fix with the exact codes from that log,
RPRT -9 and RPRT -11.

changelog.json

@@ -4,11 +4,13 @@
     "date": "",
     "en": [
       "Amplifiers: tick the ones sharing a combiner and ON, OFF and OPERATE act on all of them at once. Each keeps its own meters.",
-      "Shared CAT: with the wire trace on, every command a client sends and the answer given are logged."
+      "Shared CAT: with the wire trace on, every command a client sends and the answer given are logged.",
+      "Shared CAT: JTDX and WSJT-X no longer get an error for turning split off on a rig that has none, or for setting the transmit mode — which made JTDX abandon a transmission."
     ],
     "fr": [
       "Amplificateurs : coche ceux qui partagent un combiner et ON, OFF et OPERATE agissent sur tous à la fois. Chacun garde ses mesures.",
-      "CAT partagé : avec la trace activée, chaque commande envoyée par un client et la réponse donnée sont journalisées."
+      "CAT partagé : avec la trace activée, chaque commande envoyée par un client et la réponse donnée sont journalisées.",
+      "CAT partagé : JTDX et WSJT-X ne reçoivent plus d erreur en désactivant un split inexistant ni en réglant le mode d émission — ce qui faisait abandonner une émission à JTDX."
     ]
   },
   {