Replace a printf() with rig_debug()

The bug is that we don't know why the string is invalid.

Test case:
before
tests/rigctl -r "invalid:123:port" -m 2
Unhandled host=invalid:123:port
Unhandled host=invalid:123:port

after:
tests/rigctl -r "invalid:123:port" -m 2
(no output)
tests/rigctl -r "invalid:123:port" -m 2 -v
parse_hoststr: Unhandled host=invalid:123:port
parse_hoststr: Unhandled host=invalid:123:port

src/misc.c

@@ -2233,7 +2233,7 @@ int HAMLIB_API parse_hoststr(char *hoststr, int hoststr_len, char host[256],
 
     if (n >= 1 && strlen(dummy) == 0) { return RIG_OK; }
 
-    printf("Unhandled host=%s\n", hoststr);
+    rig_debug(RIG_DEBUG_BUG, "%s: Unhandled host=%s\n", __func__, hoststr);
 
     return -1;
 }