Move afs_pthread_setname_self to opr

Move afs_pthread_setname_self to opr

Move the functionality in afs_pthread_setname_self from libutil to
opr, in a new function opr_threadname_set. This allows us to more
easily use the routine in more subsystems, since most code already
uses opr.

Change-Id: I79d49617a19cd292a3b09ccfd9c9f319355a184e
Reviewed-on: https://gerrit.openafs.org/13655
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>