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.

Change-Id: I8d14dd0ec030e3d047f1d2e85c888c1d86e7f3c0
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>