OpenBSD: Complete implementation of afs_osi_TimedSleep

OpenBSD: Complete implementation of afs_osi_TimedSleep

The OpenBSD version of afs_osi_TimedSleep() is missing the required
afs_event structure and afs_getevent routine. This update adds them
(by borrowing a copy of the code from the FreeBSD implementation).

Reviewed-on: http://gerrit.openafs.org/4373
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit a4b8823950d0d7f3806fe9e7aed22502b72a79e4)

Change-Id: I1886cbb995a6c3e6ef3d4cc73895215d6293fe15
Reviewed-on: http://gerrit.openafs.org/4388
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>