rename afs/assert.h to afs/afs_assert.h

rename afs/assert.h to afs/afs_assert.h

provide a private assert header which does not
pollute the normal namespace for the assert header.

further changes will modify pthread operations to not use
generic assert as a "did it work", which breaks if NDEBUG is set

Change-Id: I43a95c0aa31ad2e772bce9142356fd5c5bedac6c
Reviewed-on: http://gerrit.openafs.org/2995
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>