Kernel: Merge PurgeableVMObject into AnonymousVMObject

This implements memory commitments and lazy-allocation of committed
memory.

Kernel/Syscalls/thread.cpp

@@ -61,6 +61,10 @@ int Process::sys$create_thread(void* (*entry)(void*), Userspace<const Syscall::S
     // FIXME: Do something with guard pages?
 
     auto thread = adopt(*new Thread(*this));
+    if (!thread->was_created()) {
+        // Could not fully create a thread
+        return -ENOMEM;
+    }
 
     // We know this thread is not the main_thread,
     // So give it a unique name until the user calls $set_thread_name on it