cs: avoid promoting unknown OS thread to known

Related to changes in 18a95c3ae6, but shouldn't affect the problem
that commit addressed, since Racket-level callbacks need to be
involved.

racket/src/cs/rumble/foreign.ss

@@ -1768,7 +1768,7 @@
             (loop (cdr types) (cons (ctype-host-rep type) reps) decls)))])))
 
 ;; Rely on the fact that a virtual register defaults to 0 to detect a
-;; thread that we didn't start. For a thread that we did start, a
+;; thread that we didn't start.
 (define PLACE-UNKNOWN-THREAD 0)
 (define PLACE-KNOWN-THREAD 1)
 (define PLACE-MAIN-THREAD 2)

racket/src/cs/rumble/virtual-register.ss

@@ -44,4 +44,6 @@
   ;; foreign thread that has not yet been initialized to run Rumble
   ;; and later layers:
   (when (eq? 0 (current-future))
-    (init-virtual-registers)))
+    (init-virtual-registers)
+    ;; set `place-thread-category` back to "unknown":
+    (place-thread-category PLACE-UNKNOWN-THREAD)))