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
As @asteven mentioned, $__global was undefined when I attempted to access $__global/explorer/os from within the explorer. I'll migrate to using $($__explorer/os) -- I didn't realize that was an option.