fix(udp): ignore N0CALL, and stop returning a grid as a callsign

N0CALL is what WSJT-X transmits under when its owner never set a callsign. It
has a letter, a digit and an ordinary shape, so nothing rejected it: it was
spotted, coloured, counted as a new WPX prefix, and now that CQ grids are read
it would have put a grid into the worked index under a callsign nobody holds.

The obvious fix - adding it to looksLikeCall's reject list - was wrong, and the
test caught it before it shipped. That function answers "could this token be a
callsign at all", and the CQ grammar uses it to decide whether the word after CQ
is a modifier (DX, NA, a zone) or the call itself. Teaching it that N0CALL is
not a callsign made "CQ N0CALL JN36" skip a slot and return JN36. Shape and
policy are different questions and now live in different functions.

Chasing that turned up the real defect behind it: ANY unrecognised word after CQ
made the parser skip a slot, and a four-character grid passes every shape test a
callsign does. "CQ FOO JN36" returned JN36 as the sender - logged, spotted and
coloured as a station. A grid in the callsign slot is now refused outright.

changelog.json

@@ -2,8 +2,12 @@
   {
     "version": "0.24.5",
     "date": "",
-    "en": [],
-    "fr": []
+    "en": [
+      "N0CALL and NOCALL are ignored in the digital decodes. WSJT-X transmits under N0CALL when its owner never set a callsign, and it has a perfectly ordinary shape, so it was spotted, coloured, counted as a new prefix and could bring a grid into the worked index. Fixed at the same time: an unrecognised word after CQ made the parser skip a slot, and since a grid passes every shape test a callsign does, the station grid came back as its callsign."
+    ],
+    "fr": [
+      "N0CALL et NOCALL sont ignorés dans les décodes digitaux. WSJT-X émet sous N0CALL quand son propriétaire n a jamais réglé son indicatif, et sa forme est parfaitement ordinaire — il était donc spotté, coloré, compté comme nouveau préfixe et pouvait faire entrer un grid dans l index des contacts. Corrigé au passage : un mot non reconnu après CQ faisait sauter un cran au parseur, et comme un grid passe tous les tests de forme d un indicatif, c est le grid de la station qui revenait comme son indicatif."
+    ]
   },
   {
     "version": "0.24.4",