nfsv4: Add support for the NFSv4 hidden and system attributes

There now appears to be a use for the NFSv4 hidden and system
attributes for the Windows ms-nfs41 client. As such, this
patch implements these using the UF_HIDDEN and UF_SYSTEM
flags. Commit afd5bc630930 added support for _PC_HAS_HIDDENSYSTEM,
to VOP_PATHCONF(), which is used by the server to check for
support of the UF_HIDDEN and UF_SYSTEM flags.

This patch only affects NFSv4 and only when the client/server
on the other end supports the hidden and system attributes.