Add __package_pkgng_freebsd type

Create __package_pkgng_freebsd type to handle package management via pkg-ng
Add explorer to __package to determine if "pkg" exists on the remote freebsd system
Expand logic within __package/manifest to determine which freebsd package manager to use
I wonder why you cannot access the explorers? They should be running before anything, so you can always access their value at $__global/explorer/os