Port message fragmentation supported.

- Each sendmsg() transmits no more than port->max_size payload data.
- Longer buffers are fragmented and send using multiple sendmsg() calls.
- On receive side, buffers are connected in chain.
- Number of handler calls is the same as number of nxt_port_socket_write()
  calls.
- nxt_buf_make_plain() function introduced to make single plain buffer from
  the chain.

src/nxt_sendbuf.h

@@ -58,6 +58,8 @@ typedef struct {
     uint32_t     nmax;
     uint8_t      sync;   /* 1 bit */
     uint8_t      last;   /* 1 bit */
+    uint8_t      limit_reached;
+    uint8_t      nmax_reached;
 
     size_t       size;
     size_t       limit;