r23112: Trim down the message.c API slightly: The messages_pending_for_pid is now

r23112: Trim down the message.c API slightly: The messages_pending_for_pid is now
replaced by MSG_FLAG_LOWPRIORITY or'ed into the msg_type. To enable this,
changed the msg_type definitions to hexadecimal.

This way we could theoretically add the MSG_FLAG_NODUPLICATES again, but I
would rather not do this, because that one is racy and can't be guaranteed at
all.
(This used to be commit 3f5eb8a9600839a9f9c44c553f0bda6df22b30b0)