NXT_PORT_MSG_LAST - mark message as last;
NXT_PORT_MSG_CLOSE_FD - close fd right after send;
Type constants altered to include last flag for single buffer messages.
Last sign is critical for coming port RPC layer. Handlers unregistered on last
message. Create sync buffer is not convenient, extra parameter is better.
New port message type introduced NXT_PORT_MSG_REMOVE_PID. Default handler
removes process description from nxt_runtime_t with all ports, incoming and
outgoing mmaps etc.
