config: Refactor the FreeBSD configuration files

Use a common file for most of the platform specific settings.

Change-Id: If95ad44de99fc3320570d53d706fed4d760fe67d
Reviewed-on: http://gerrit.openafs.org/11346
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Garrett Wollman <wollman@csail.mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>