Set the XATTRSUPPORT attribute bit for NFSv4.2, always cleared for now.

Since r355472 added code which clears the XATTRSUPPORT bit for non-NFSv4.2
mounts, it is now safe to set it.

There will be a series of these preliminary commits that will prepare for
a major commit of the NFSv4.2 client/server changes currently found in
subversion under projects/nfsv42/sys.
This commit completes updates to nfsproto.h required by the NFSv4.2.