Logfiles: open with O_APPEND

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.

Change-Id: I09437aac63205fee3d97850507531e6833fed14f
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>