Rewrite clnt_vc_soupcall() so that it does soreceive() for an
entire TLS record (or all data available for TCP without TLS), similar
to what soreceive() in svc_vc.c does.
This should avoid problems if/when an NFS RPC message record mark
straddles TLS records and reduces the number of soreceive() calls
done by the client.

It also makes adding code to handle the TLS control records simpler.
This is next on my todo list.