This allows us to clear the area around the null pointer, and so trap errors.

The switch is tricky wrt setting up memory, since we need the memory to
be mapped in order to free it, but we need free memory to map (for the page
tables).  What we do is set up a temporart 1:1 map at ffff800000000000
that mirrors the first 1G map we already have from bootstreap, use that
to free all of memory under 1G, then map and free everything else.

Initial memory is physical; the mmu converts it to virtual addresses, and
then it can be added to the memory pool.  Right now there is not much
difference, but the 1:1 mapping is moving soon.

Reserves a vma range for later use.  Can be useful to implement mmap() hints
with a two-step algorithm - reserve a range, then overwrite it with the real
mmap.

This is simpler than [addr, addr+size) for splitting and merging.

These sections have memsz > filesz.  This means we need separate
map_file() and file_anon() with caller-specified addresses.