Simplify VM and UMA startup by eliminating boot pages. Instead use careful
ordering to allocate early pages in the same way boot pages were but only
as needed. After the KVA allocator has started up we allocate the KVA that
we consumed during boot. This also makes the boot pages freeable since they
have vm_page structures allocated with the rest of memory.

Parts of this patch were written and tested by markj.

Reviewed by: glebius, markj
Differential Revision: https://reviews.freebsd.org/D23102