Logfiles: open with O_APPEND

This does not change the current (normal) behaviour, but allows
logrotation via "copy and truncate" as offered by logrotate.
Otherwise the processes will remember the offset of the last write
and a truncated file is filled with '\0' until the current offset.
The mrafsStyleLogs are untouched, since they can be rotated by a
kill -HUP and are deprecated anyway.

Reviewed-on: http://gerrit.openafs.org/11092
Reviewed-by: Nathaniel Filardo <nwfilardo@gmail.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit b71a041364d28d6a56905a770cd20d1497ee26ec)

Change-Id: Idc148f4d6b9302d25cbf32763d08ee24eaf7378b
Reviewed-on: http://gerrit.openafs.org/11193
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>