Review flags that are supplied (not only) from user space

[fridex]

Review flags that can be supplied (e.g. MSG_NONBLOCK, MSG_MORE, etc.). Some of them are not useful for TLS/DTLS, but some are. Some ideas:

• MSG_DONTWAIT - for sendmsg(2) can be implemented with a worker
• MSG_MORE - has to store previous bytes - sendpage() context can be reused for
  this purpose
• MSG_NOSIGNAL - can be propagated when suitable
• MSG_PEEK -this will require not to clear cached record if received
  asynchronously or write to cache when waited for data (in
  kernel_recvmsg() rx queue)
• MSG_WAITALL - this can be tricky, a use case when a user supplies more than
  TLS_MAX_PAYLOAD_SIZE (1 << 14) should be not supported from my POV
• ...

[fridex]

• MSG_ERRQUEUE could be used to explicitly probe for encrypted record causing error (not to read it from bound socket) - this is overkill for uses pace to handle and I don't like it though

[djwatson]

#84 should handle MSG_DONTWAIT, MSG_MORE. MSG_PEEK should already be implemented. I agree MSG_ERRQUEUE and MSG_WAITALL don't make much sense. I think the sk_wait handling already takes care of MSG_NOSIGNAL