Make this compile on 7.2 again

The important points to note is that __FreeBSD_version is now being
used to pick out when features were added with appropriate HAS_*
macros in the .c file.

Sponsored-by: EMC Isilon
Refine the filesystem printing check for FreeBSD

Sponsored-by: EMC Isilon
Factor out determining the mountpoint into its own function

Do the same for the "unsupported OS" message