Kernel: Slap UNMAP_AFTER_INIT on a whole bunch of functions

There's no real system here, I just added it to various functions
that I don't believe we ever want to call after initialization
has finished.

With these changes, we're able to unmap 60 KiB of kernel text
after init. :^)

Kernel/Process.cpp

@@ -78,7 +78,7 @@ ProcessID Process::allocate_pid()
     return next_pid.fetch_add(1, AK::MemoryOrder::memory_order_acq_rel);
 }
 
-void Process::initialize()
+UNMAP_AFTER_INIT void Process::initialize()
 {
     g_modules = new HashMap<String, OwnPtr<Module>>;