FBSD: Remove support for 8.x and 9.x

FBSD: Remove support for 8.x and 9.x

According to <https://www.freebsd.org/security/unsupported.html>,
FreeBSD 8.x EoL was on August 1, 2015, and FreeBSD 9.x EoL was on
December 31, 2016. Remove our support for these versions, since they
haven't been supported by FreeBSD itself for a while.

FreeBSD 10.x EoL was on October 31, 2018, which has passed, but was
less than a year ago. So keep 10.x in for now.

Adjust our preprocessor checks accordingly:

- In FBSD-specific dirs, assume AFS_FBSD100_ENV and lower is always
  true. Assume __FreeBSD_version is always at least 1000000.

- In non-FBSD dirs, convert AFS_FBSD100_ENV and lower to AFS_FBSD_ENV.

Change-Id: I965e65d3b95573bb374661217b24b686c7b68ed2
Reviewed-on: https://gerrit.openafs.org/13842
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>