nfs: Add support for the NFSv4 archive attribute

The archive attribute (UF_ARCHIVE) is listed as deprecated
in RFC8881. However, the Windows NFSv4.1 client folk disagree
with this and say they need support for it.

This patch adds support for it, in a manner similar to what
is done for UF_SYSTEM and UF_HIDDEN.

MFC after: 2 weeks