Suport for multiple ports repos.

We will use this for zfsd and other TrueNAS addons.

Conflicts:
	nanobsd/common
Bring in git for all builds.

We will use this to fetch out nas_ports.
set NAS_PORTS directory properly using AVATAR_ROOT

Conflicts:
	nanobsd/os-base
Hide zfsd under NANO_LABEL = TrueNAS, fix port path
If building TrueNAS use private package source.
Mount extra ports under ports_dir, main ports under ports.

We need the Mk files from "FreeBSD ports" so we ALWAYS
nullfs mount freebsd ports to /usr/ports, any other ports
directory is mounted to /usr/ports_dir.
set PORTDIR correctly for Mk file inclusion.