Update NEWS for 1.5.78 and 1.6.0pre1

Update NEWS for 1.5.78 and 1.6.0pre1

Based on the release notes.  Add an entry for 1.6.0 with in progress
instead of a date to hold the release notes for 1.6.0pre1 rather than
creating separate entries for each release candidate.  Users who track
the release candidates can refer to the public announcements, and this
will be cleaner when reading history later on.

Reviewed-on: http://gerrit.openafs.org/3592
Tested-by: Russ Allbery <rra@stanford.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 676a4efc232bc77fe20e46c2dbe45e89f89cd670)

Change-Id: I324e55c82bf2e353143368ccd4ca074a646d11d1
Reviewed-on: http://gerrit.openafs.org/3594
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>