Free memory from afs_events

Free memory from afs_events

DARWIN and LINUX were already doing this, but everybody else had
a memory leak.  Consolidate most of the common code to do so,
including afs_event_t definitions.

Change-Id: I5ec83cf23fd15dbbd5716995c671998e35862843
Reviewed-on: http://gerrit.openafs.org/5314
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@dementix.org>