nfscl: Add support for a NFSv4 AppendWrite RPC

For IO_APPEND VOP_WRITE()s, the code first does a
Getattr RPC to acquire the file's size, before it
can do the Write RPC.

Although NFS does not have an append write operation,
an NFSv4 compound can use a Verify operation to check
that the client's notion of the file's size is
correct before doing the Write operation.

This patch prepares the NFSv4 client for such an
RPC, which will be added in a future commit.

This patch does not cause any semantics change.