Set UDP datagram source address (ticket #1239).

Previously, the source IP address of a response UDP datagram could differ from
the original datagram destination address.  This could happen if the server UDP
socket is bound to a wildcard address and the network interface chosen to output
the response packet has a different default address than the destination address
of the original packet.  For example, if two addresses from the same network are
configured on an interface.

Now source address is set explicitly if a response is sent for a server UDP
socket bound to a wildcard address.

auto/unix

@@ -394,6 +394,19 @@ ngx_feature_test="setsockopt(0, IPPROTO_IP, IP_RECVDSTADDR, NULL, 0)"
 . auto/feature
 
 
+# BSD way to set IPv4 datagram source address
+
+ngx_feature="IP_SENDSRCADDR"
+ngx_feature_name="NGX_HAVE_IP_SENDSRCADDR"
+ngx_feature_run=no
+ngx_feature_incs="#include <sys/socket.h>
+                  #include <netinet/in.h>"
+ngx_feature_path=
+ngx_feature_libs=
+ngx_feature_test="setsockopt(0, IPPROTO_IP, IP_SENDSRCADDR, NULL, 0)"
+. auto/feature
+
+
 # Linux way to get IPv4 datagram destination address
 
 ngx_feature="IP_PKTINFO"