util: simplify thread-name interface

util: simplify thread-name interface

It appears that we don't actually need an interface to set the name
of an arbitrary thread (which Mac OS can't do), so remove the
afs_pthread_setname() interface and promote afs_pthread_setname_self()
to the status of primary.

Change-Id: I2d915d8165dac9ccfe0cb99630db657cb1473389
Reviewed-on: http://gerrit.openafs.org/5121
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>