nfscl/kgssapi: Fix Kerberized NFS mounts to pNFS servers

During recent testing related to the IETF NFSv4 Bakeathon, it was
discovered that Kerberized NFSv4.1/4.2 mounts to pNFS servers
(sec=krb5[ip],pnfs mount options) was broken.
The FreeBSD client was using the "service principal" for
the MDS to try and establish a rpcsec_gss credential for a DS,
which is incorrect. (A "service principal" looks like
"nfs@<fqdn-of-server>" and the <fqdn-of-server> for the DS is not
the same as the MDS for most pNFS servers.)

To fix this, the rpcsec_gss code needs to be able to do a
reverse DNS lookup of the DS's IP address. A new kgssapi upcall
to the gssd(8) daemon is added by this patch to do the reverse DNS
along with a new rpcsec_gss function to generate the "service
principal".

A separate patch to the gssd(8) will be committed, so that this
patch will fix the problem. Without the gssd(8) patch, the new
upcall fails and current/incorrect behaviour remains.

This bug only affects the rare case of a Kerberized (sec=krb5[ip],pnfs)
mount using pNFS.

This patch changes the internal KAPI between the kgssapi and
nfscl modules, but since I did a version bump a few days ago,
I will not do one this time.

MFC after: 1 month