Make it possible to override PAE mode on boot.

Initialize the static kenv in pmap_cold() and fetch user opinion on
vm.pmap.pae_mode tunable if hardware is capable. Note that the static
environment is reinitilized in init386() later when paging is enabled.

Reviewed by: bde
Discussed with: kevans
Sponsored by: The FreeBSD Foundation
MFC after: 2 months