SOLARIS: Avoid areq and auid conflict

On new Solaris (11.1), nfs/auth.h #defines areq and auid to access
some elements inside the nfsauth_arg structure more easily. We have a
lot of functions that use those names as parameters, so the compiler
throws an error (since we have a decl like "struct vrequest
*areq_u.areq").

We cannot avoid including that header, since we need some NFS-related
headers for the NFS xlator, and they pull in nfs/auth.h
unconditionally. So, work around this by undefining areq and auid
afterwards.

Reviewed-on: http://gerrit.openafs.org/8873
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 936c2b02bf6fbeb3844c539665392ae89eb652fe)

Change-Id: I54da96a48d163b6aa57bb8525ff4c80f10d70fa1
Reviewed-on: http://gerrit.openafs.org/8889
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>