LibPthread: Add magic bytes to the start of sem_t structures

This helps ensure random pointers are not passed in as semaphores, but
more importantly once named semaphores are implemented, this will
ensure that random files are not used as semaphores.

Userland/Libraries/LibC/semaphore.h

@@ -14,6 +14,7 @@
 __BEGIN_DECLS
 
 typedef struct {
+    uint32_t magic;
     uint32_t value;
 } sem_t;