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.

Change-Id: Ifd139917dfe7a1221941351f7873fe183c617159
Reviewed-on: http://gerrit.openafs.org/8873
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>