afs: Define afs_warnall routine

In a Linux environment afs_warn and afs_warnuser both go to
the same spot, resulting in duplicated messages if both are
invoked back to back.  Define a new function afs_warnall
for use when identical messages are directed to both warn
and warnuser.  In a Linux environment it will do the right
thing and present only one copy of the message.

Reviewed-on: http://gerrit.openafs.org/10943
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit a70f8e1a7e3fcbc13c9a60f8a7409dd01d014afb)

Change-Id: I264633cd4a30cfb7f7264e56512f318cfb23ef3e
Reviewed-on: http://gerrit.openafs.org/11182
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>