- Add priority message queue to msgport. Send a message with MSGF_PRIORITY

- Add priority message queue to msgport.  Send a message with MSGF_PRIORITY
  flag will queue the message into the priority message queue of the target
  port.  The priority message queue takes precendence over normal message
  queue, so the messages with MSGF_PRIORITY flag will be processed before
  other messages on the same target port.  This could be used by defering
  callout or operation that should not be delayed too long on the target
  port.
- Add dropmsg function to msgport.  Message must be marked with MSGF_DROPABLE,
  else dropmsg operation is not allowed.  Message marked with MSGF_DROPABLE
  is not waitable, i.e. you could not call domsg on this kind of message.
  Currently only thread msgport supports this operation and this operation
  must be performed in the same thread of the msgport's owner thread.

Discussed-with: dillon@