Using shared memory to send data via nxt_port.

Usage:
    b = nxt_port_mmap_get_buf(task, port, size);
    b->mem.free = nxt_cpymem(b->mem.free, data, size);
    nxt_port_socket_write(task, port, NXT_PORT_MSG_DATA, -1, 0, b);

src/nxt_sendbuf.h

@@ -53,6 +53,7 @@ typedef struct {
 typedef struct {
     nxt_buf_t    *buf;
     nxt_iobuf_t  *iobuf;
+    nxt_uint_t   niov;
 
     uint32_t     nmax;
     uint8_t      sync;   /* 1 bit */