Userspace dlmalloc port, supporting syscalls mman_map()/mman_unmap()

kernel/pmm/pmm.c

@@ -57,7 +57,7 @@ void *pmm_alloc(size_t pages) {
 
   if (!phys) {
     ERR("pmm", "phys memory ran out\n");
-    hal_hang();
+    return NULL;
   }
   return (void *)phys;
 }