Add a couple of definitions for NFSv4.2 and update macros to use them.

This patch adds code to macros to clear attribute bits not supported
by NFSv4.2. For now, these bits are never set anyhow, but this prepares
the code for the addition of NFSv4.2 support in a future commit.

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.