Implement mprotect(). There's still one missing feature though - PROT_NONE

is not currently supported, and PROT_READ is used instead (i.e., instead of
inaccessible memory, one gets read-only memory). Inaccessible memory
complicates the code because it means using the non-present bit on
still-allocated pages, and then unpopulate() can't use the present bit to
decide which pages are allocated and need to be freed.
To fix this, I'll need to:
1. unpopulate_page() and unpopulate_huge_page() should assume it is only
ever called on allocated pages (evacute() actually ensures that!), and not
return on non-present ptes.
2. change_perm() should use the read_perm to set the present bit.
3. protect() shouldn't rely on present bit to know when to return 0 or 1,
   rather it must be wrapped in something like evacute() which knows to
   find mapped ranges, and only call protect() for actually mapped ranges.
I'll try this tomorrow.