Implement kvm_kerndisp

This change adds a new libkvm function, kvm_kerndisp(), that can be used to
retrieve the kernel displacement, that is the difference between the kernel's
base virtual address at run time and the kernel base virtual address specified
in the kernel image file.

This will be used by kgdb, to properly relocate kernel symbols, when needed.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D23285