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/Devices/NullDevice.cpp

@@ -42,12 +42,12 @@ NullDevice& NullDevice::the()
     return *s_the;
 }
 
-NullDevice::NullDevice()
+UNMAP_AFTER_INIT NullDevice::NullDevice()
     : CharacterDevice(1, 3)
 {
 }
 
-NullDevice::~NullDevice()
+UNMAP_AFTER_INIT NullDevice::~NullDevice()
 {
 }