fix(cat-share): answer get_freq with the listening VFO, not the transmit one

Reported from a station running an IC-7850 over USB: with split engaged,
turning VFO B moved VFO A slightly. OpsLog writes nothing to that radio on
its own — but it shares it, and what it was sharing was wrong.

Hamlib's 'f' means the VFO IN USE. RigState follows ADIF, where FreqHz is
where we TRANSMIT, and the two are the same number until split is
engaged — so the adapter handed FreqHz over unchanged and every client
asking for the dial was given VFO B. A client that reads the dial and
writes it back, which is what WSJT-X and its like do, then wrote VFO B's
frequency into VFO A. Hence a shift the size of the split offset, and
hence 'it did not do this before': it only happens with split on.

get_split_freq ('i') already answers the transmit frequency and is
untouched. The one-line rule is lifted into shareRXFreq with a test that
includes this case, the simplex case, and a backend that reports split
without ever filling in the receive frequency — which would otherwise
answer a client with 0 Hz.

changelog.json

@@ -4,11 +4,13 @@
     "date": "",
     "en": [
       "Awards, RDA district comparison: each disagreement can now be settled on its own row. Click the district you keep — the log's or the database's — and apply. It is written into the contact's CNTY and its award reference, and the settled rows leave the list.",
-      "The band/mode matrix: a confirmed slot now shows as confirmed. A callsign worked and not confirmed was outranking an entity CONFIRMED on the same band and mode, so a slot that needs nothing was painted as if it still did."
+      "The band/mode matrix: a confirmed slot now shows as confirmed. A callsign worked and not confirmed was outranking an entity CONFIRMED on the same band and mode, so a slot that needs nothing was painted as if it still did.",
+      "CAT sharing: a program connected to OpsLog's rigctl server is told the frequency it is LISTENING on. With split engaged it was given the transmit VFO instead, so a client that reads the dial and writes it back — WSJT-X and its like — could move VFO A when VFO B was turned."
     ],
     "fr": [
       "Diplômes, comparaison des districts RDA : chaque divergence se règle désormais sur sa propre ligne. Cliquer le district qu'on garde — celui du log ou celui de la base — puis appliquer. Il est écrit dans le CNTY du contact et dans sa référence de diplôme, et les lignes réglées quittent la liste.",
-      "Matrice bandes/modes : une case confirmée s'affiche enfin comme confirmée. Un indicatif travaillé et non confirmé l'emportait sur une entité CONFIRMÉE sur la même bande et le même mode, si bien qu'une case qui ne demandait plus rien était peinte comme s'il manquait encore quelque chose."
+      "Matrice bandes/modes : une case confirmée s'affiche enfin comme confirmée. Un indicatif travaillé et non confirmé l'emportait sur une entité CONFIRMÉE sur la même bande et le même mode, si bien qu'une case qui ne demandait plus rien était peinte comme s'il manquait encore quelque chose.",
+      "Partage CAT : un programme connecté au serveur rigctl d'OpsLog reçoit la fréquence sur laquelle on ÉCOUTE. En split, c'était le VFO d'émission qui lui était donné, si bien qu'un client qui lit le VFO et le réécrit — WSJT-X et consorts — pouvait déplacer le VFO A quand on tournait le VFO B."
     ]
   },
   {