auth: Simplify DNS lookups with asprintf

auth: Simplify DNS lookups with asprintf

Instead of allocing a maximal string, and using snprintf to
construct each possible DNS search string, just use asprintf to
construct each string. This greatly simplifies the code, and makes
it much less likely that maths errors can creep in causing buffer
overflows in the future. The downside is that we have more round
trips to the allocator, but that shouldn't matter in this context.

Change-Id: Iae9ab7d45f454c90a937354c71c9ec7fa2c55b69
Reviewed-on: http://gerrit.openafs.org/7452
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>