config: Updates to AFS_HAVE_STATVFS for FreeBSD

Always define AFS_HAVE_STATVFS.  According to the man page, statvfs()
appeared in FreeBSD 5.0.  Additionally, this macro is only used for
userspace which eliminates all disables except for FreeBSD 5.0 which
appears to have just been an oversight when the param file was created
from the 4.x param files.

Also fixes the comment so it reflects the actual choice.

Change-Id: Ibdcd694e9c4a0d0cecccd91a51962af6fb11ff36
Reviewed-on: http://gerrit.openafs.org/11344
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Garrett Wollman <wollman@csail.mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>