Adjust configure defaults for 1.8

Adjust configure defaults for 1.8

Disable pam; enable pthreaded ubik.
(Pam is actually not installed by default anyway, since it is
functionally part of kauth.)

pthreaded ubik is believed to be stable, and we want to move away
from LWP moving forward.

Change-Id: I14d20e3157df625a9e04059534bbb59268384213
Reviewed-on: http://gerrit.openafs.org/11483
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>