cs: fix call-in-os-thread to not retain the current Racket thread

Closes #3843

pkgs/racket-test/tests/racket/os-thread.rkt

@@ -0,0 +1,30 @@
+#lang racket/base
+(require ffi/unsafe
+         ffi/unsafe/os-thread
+         ffi/unsafe/os-async-channel)
+
+(when (os-thread-enabled?)
+  (define ch1 (make-os-async-channel))
+  (define ch2 (make-os-async-channel))
+  
+  (define wb
+    (make-weak-box
+     (thread (lambda ()
+               (call-in-os-thread
+                (lambda ()
+                  (os-async-channel-put ch1 'ready)
+                  (os-async-channel-get ch2)))
+               (sync never-evt)))))
+
+  (sync (system-idle-evt))
+  (os-async-channel-get ch1)
+
+  (collect-garbage)
+
+  (when (weak-box-value wb)
+    (error "thread was retained"))
+
+  (os-async-channel-put ch2 'done)
+
+  'success)
+

racket/src/cs/schemified/thread.scm

@@ -15049,7 +15049,11 @@
          (if threaded? (void) (raise-unsupported 'unsafe-call-in-os-thread))
          (|#%app|
           fork-pthread
-          (lambda () (begin (start-atomic) (|#%app| proc_0))))
+          (lambda ()
+            (begin
+              (start-atomic)
+              (current-thread/in-atomic #f)
+              (|#%app| proc_0))))
          (void))))))
 (define finish_2628
   (make-struct-type-install-properties

racket/src/thread/os-thread.rkt

@@ -1,7 +1,8 @@
 #lang racket/base
 (require "check.rkt"
          "host.rkt"
-         "atomic.rkt")
+         "atomic.rkt"
+         "parameter.rkt")
 
 (provide unsafe-os-thread-enabled?
          unsafe-call-in-os-thread
@@ -17,6 +18,7 @@
   (unless threaded? (raise-unsupported who))
   (fork-pthread (lambda ()
                   (start-atomic) ; just in case
+                  (current-thread/in-atomic #f) ; don't inherit
                   (proc)))
   (void))