Various fixes for 'kvm' for FreeBSD 11.
Various fixes for 'kvm' for FreeBSD 11.

This includes the submap functions needing to use $arg0 instead of
$_submap.

While here, also add a not-yet-tested vmem command.

The kvm map on 11 is a bit less useful as the initial map entry grows
to include kernel_arena allocations on riscv at least.

bsdjhb/kdbg,bsdjhb/kdbg