CPack-FreeBSD: add a generator for FreeBSD pkg(8)
Adds a CPack generator that produces native FreeBSD packages (and possibly DragonflyBSD, too; the packaging library is also available on Linux, but I don't think anyone uses it as their native package format).
This is likely to go in to the FreeBSD packaged CPack regardless, but it'd be nice to have it upstream as well.
License: 3-clause BSD (it says so in the files, too, but I'll make it explicit in the merge-request).