Add a sysctl to dump kernel mappings and their properties on amd64.

The sysctl is called vm.pmap.kernel_maps. It dumps address ranges
and their corresponding protection and mapping mode, as well as
counts of 2MB and 1GB pages in the range.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21380