Fix gdt entry generation so that limit works properly
Fix up isr stub to return control properly (i.e don't fuck up the stack)
Remove some debug messages
Fiddle with physical allocator stuff. Kernel now has a dumb placement allocator from 4MB, with a smarter heap starting at 16MB
A bit of cleanup and output formatting
Fix paging to work and map lower 16MB as kernel rather than user
Load userland at 0xc0000000, mostly to test paging.