Implement constrained direct memory writes

[fwsGonzo]

After two days of hammering the keyboard and getting increasingly annoyed, I've finally made constrained working memory for main VM work. It turned out better than I thought because it has some nice properties. Here's a short explanation:

• The main VM is turned entirely copy-on-write but importantly with no other pagetable changes. That is, every writable page is turned into read-only + cloneable. The equivalent of iterating the page table and toggling bits.
• The main VM has a property called "direct main memory writes" enabled.
• When a read-only page is being written to, a page fault happens that enters paging. Paging sees that it's a VM with direct memory writes, and just unlocks the page by making it writable again. No other change, making it very fast. It also counts the pages that are unlocked. And if the number of unlocked pages exceeds the set working memory, it throws an out of memory exception.

When the VM has been loaded, it is again made copy-on-write and any forks will see the pagetables in the same way as before (without this feature). So, bottom line is: Not a single extra page is used, but we can still constrain pages used.