nfscl: Add support for the NFSv4.1/4.2 WANT_xxx flags

NFSv4.1/4.2 defined new OPEN_WANT_xxx flags that a client
can use to hint to the server that delegations are or are
not wanted. This patch adds use of those delegations to
the client.

This patch should only affect the NFSv4.1/4.2 behaviour
when delegations are enabled, which is not the default.

MFC after: 1 month