dskPercentNode support for FreeBSD4

Here's a patch (againt todays cvs) to add
dskPercentNode for (at least) FreeBSD.

Freebsd doesn't have a statvfs function or a f_favail
(free inodes avail to non-superuser)
structure member.

I've tried to make the patch generic so it _may_ work
on other platforms that also only have a statfs function.

(FreeBSD layout seems unusual - statfs struct is
defined in sys/mount.h)

I've read PORTING doc but this is my first attempt at
using autoconf tools and "system" programming so
forgive me if the patch is not 100%.
Any tips/feedback would be welcome.

All I can say it that it works for me on FreeBSD46 :-)

Hope this helps and thank you for providing such a
useful tool.

Donal