Use stable output to a test file instead of depending on the OS name being
grep'able in /bin/sh

This fixes the situation where the OS has been rebranded to something other
than `FreeBSD`

MFC after: 1 week
Obtained from: Isilon OneFS (^/onefs/head@r511419)
Reviewed by: cem, Daniel O'Connor <darius@dons.net.au>
Sponsored by: EMC / Isilon Storage Division