SystemInformation: Treat BSDs more uniformly

Take changes used by pkgsrc [1]:

* Add more conditional handling for NetBSD, same as others.
* Treat FreeBSD and DragonFly the same way as NetBSD and OpenBSD.

[1] http://cdn.netbsd.org/pub/pkgsrc/current/pkgsrc/devel/cmake/patches/

Change-Id: If94a931dd1609565459eb9891561f341cc990bf0