Do not reuse shared memory segment with different port until this segment
successfully received and indexed on other side. However, segment can be used
to transfer data via the port it was sent at any time.
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.
There is a case in router where we use port in router connection thread.
Buffers are allocated within connection memory pool which can be used only in
this router thread. sendmsg() can be postponed into main router thread and
completion handler will compare current engine and post itself to correct
engine.
nxt_port_mmap_t stored in arrays and it is unsafe to store
pointer to array element.
Shared memory structures and macros moved to separate header
file to be used by GO package.
