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'd suggest to use the output of command -v directly. if the output is empty, then there is no pkg-ng. I especially recommend to *not* use values with spaces :-)