Unix CM: Fix afs_event fallout on Mac OS X

Unix CM: Fix afs_event fallout on Mac OS X

Commit dc077b83c9b1f107efbc3483743f44117748f23c moved the afs_event
structure definition into afs_osi.h. However, afs_osi.h is included from
lots of places which don't include a complete set of platform headers.
This means that the use of lck_mtx_t in this file breaks the build on
Darwin.

Instead, define the structure in afs.h, along with all of the rest of
the cache manager structures. Also take the opportunity to move the
definition of shutdown_osisleep it afs_prototypes.h, with the rest of
the prototypes.

Change-Id: Idd19fc30adcb5ab37e65ad87fa9224668e8cd321
Reviewed-on: http://gerrit.openafs.org/5392
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>