rework PTE bits

the least significant 12 bits are ignored

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
max page table size is 256Kb

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
make use of a bitmap to keep track of free pages

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
we have a page size of 4k

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>