POSIX: shm_open: set the FD_CLOEXEC file descriptor flag...

on the new file descriptor.
Fixes #13774.

src/system/libroot/posix/sys/mman.cpp

@@ -178,7 +178,7 @@ shm_open(const char* name, int openMode, mode_t permissions)
 	if (error != B_OK)
 		RETURN_AND_SET_ERRNO(error);
 
-	return open(path, openMode, permissions);
+	return open(path, openMode | FD_CLOEXEC, permissions);
 }