Provide an abstract work queue object

Provide an abstract work queue object

Add some routines for specifying chunks of work to be done. The idea
is to be able to pass these to different threads, and specify
dependencies between them, wait on them completing, etc.

This adds the afs_wq* family of functions. Originally written by Tom
Keiser.

Change-Id: If556cf4da12de8c4be1e53376d85d791584ae177
Reviewed-on: http://gerrit.openafs.org/1862
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>