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
Nico Schottelius wrote on 09/17/2014 09:18 PM:
>> [..]
> 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

Not quite. This runs on the target host, so you don't have access to the 
value. But you can execute the other explorers like so:

value=$($__explorer/os)

vs

value=$(cat $__global/explorer/os)

as you would use in manifest and gencode and friends.