Make afs_warn() and afs_warnuser() va_args

Make afs_warn() and afs_warnuser() va_args

The afs_warn() and afs_warnuser() functions take a variable number of
arguments. Historically, we've handled this by just not prototyping
them. This change builds on the work done a while back to get a
prototyped osi_Panic() working in the kernel, and contains the same
work arounds for platforms with no usable va_args support (HPUX) and
those where support is limited (some Darwin, some Linux)

Reviewed-on: http://gerrit.openafs.org/27
Reviewed-by: Russ Allbery <rra@stanford.edu>
Verified-by: Russ Allbery <rra@stanford.edu>
Verified-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>