Update NEWS for 1.6.10

Add the change number of the late aklog fix. Also state that we
support Linux clients up to 3.16 only, since we already know that
3.17 needs a few tweaks, probably to ship with 1.6.10.1 .

Change-Id: I5904d366611e329dbf06e1b6f2ba3ebf366e7d05
Reviewed-on: http://gerrit.openafs.org/11531
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>