viced: Abstract out hpr thread context code

viced: Abstract out hpr thread context code

Every hpr lookup function had a copy of the same code to pull a thread
specific ubik context out of the pthread library, and to create one if
one wasn't already there.

Instead of endlessly repeating this code, create a small inline function
to do the job.

Change-Id: I7eac0eaebbd42978e40c90f0ce59b26f2e67d96d
Reviewed-on: http://gerrit.openafs.org/7558
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>