afs: restructure afs_warn and afs_warnuser

Restructure afs_warn.c to provide an afs_vwarn and afs_vwarnuser
that accepts a va_list rather than a variadic parameter
specification.  afs_warn and afs_warnuser continue to be
variadic functions but now call afs_vwarn and afs_vwarnuser.
This is a preparatory change.  A subsequent update will
further exploit afs_vwarn and afs_vwarnuser.

Reviewed-on: http://gerrit.openafs.org/10942
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 c3d4c109305b2db8a63b754c1894ad37326dc340)

Change-Id: Ica9c3c8a0ae2c5b680b2e11bc85c747cfd11d30a
Reviewed-on: http://gerrit.openafs.org/11181
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>