Add a port for the Solaris Modular Debugger (mdb).

Currently it only supports debugging of FreeBSD/amd64 kernels via kernel
crashes (vmcores) or live systems via /dev/mem. It does not include a
port of the in-kernel kmdb debugger.

PR: 220891
Reviewed by: mat
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D11682