auth: local realms configuration

auth: local realms configuration

Add krb.conf and krb.excl support to the auth cell configuration
library.  Provide a function to determine if the user is local to the
cell.  Provide a function to set the local realms during application
initialization.  These changes are intended to replace the functions
afs_krb_get_lrealm and afs_is_foreign_ticket_name.

Change-Id: Iba57e9ffc2c958f3a4565a9352ce172189276ce9
Reviewed-on: http://gerrit.openafs.org/5744
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>