Linux: Refine openafs-client.service dependencies

Make sure that openafs-client is started after the network is online,
and before services that need remote-fs at startup.

Change-Id: Ib69e0c07f9079f47bbe5057626652c44039c433a
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/11441
Reviewed-by: Chaskiel Grundman <cg2v@andrew.cmu.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>