Add support for struct sockaddr with no sa_len field.

Lwip's struct sockaddr includes sa_len, but some systems
like Linux doesn't have this filed, which produces many
compilation problems when using external headers.

A set of macros has benn added to detect the absence of
sa_len and adapt sockets.c

src/include/lwip/sockets.h

@@ -59,6 +59,9 @@
 extern "C" {
 #endif
 
+/* sockaddr and pals include length fields */
+#define _HAVE_SA_LEN  1
+
 /* If your port already typedef's sa_family_t, define SA_FAMILY_T_DEFINED
    to prevent this code from redefining it. */
 #if !defined(sa_family_t) && !defined(SA_FAMILY_T_DEFINED)