Introduce __package_pkgng_freebsd type

Create __package_pkgng_freebsd type to handle package management via pkg-ng
Add requirement generation in __package/manifest
Add explorer inside __package to find if pkg-ng is installed on the target
Minor fixes

Fixed pkgng_exists explorer (can't reference $__global)
Fixed logic in __package/pkgng_freebsd/gencode-remote
   Wasn't installing package if it already exists, but after explorer execution
Implemented telmich's suggestions

Using $($__explorer/os), command -v directly
Fix spacing
Fix copyright year
Merge remote-tracking branch 'jake/__pkgng'
+__package_pkgng_freebsd

Signed-off-by: Nico Schottelius <nico@freiheit.schottelius.org>