afs: restructure afs_warn and afs_warnuser

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.

Change-Id: I8e740c4db311582bda6422e6600f1503dfbd0f5a
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>