[build] Avoid strict-aliasing warnings when building with gcc 4.4

Conventional usage of the various struct sockaddr_xxx types involves
liberal use of casting, which tends to trigger strict-aliasing
warnings from gcc.  Avoid these now and in future by marking all the
relevant types with __attribute__((may_alias)).

src/include/gpxe/in.h

 	char pad[ sizeof ( struct sockaddr ) - sizeof ( sa_family_t )
 					     - sizeof ( uint16_t )
 					     - sizeof ( struct in_addr ) ];
-};
+} __attribute__ (( may_alias ));
 
 /**
  * IPv6 socket address
         uint32_t        sin6_flowinfo;  /* Flow number */
         struct in6_addr sin6_addr;      /* 128-bit destination address */
         uint32_t        sin6_scope_id;  /* Scope ID */
-};
+} __attribute__ (( may_alias ));
 
 extern int inet_aton ( const char *cp, struct in_addr *inp );
 extern char * inet_ntoa ( struct in_addr in );

src/include/gpxe/open.h

 
 #include <stdarg.h>
 #include <gpxe/tables.h>
+#include <gpxe/socket.h>
 
 struct xfer_interface;
 struct uri;
-struct sockaddr;
 
 /** Location types */
 enum {

src/include/gpxe/resolv.h

 #include <gpxe/refcnt.h>
 #include <gpxe/interface.h>
 #include <gpxe/tables.h>
+#include <gpxe/socket.h>
 
-struct sockaddr;
 struct resolv_interface;
 
 /** Name resolution interface operations */

src/include/gpxe/socket.h

 	 * family.
 	 */
 	char pad[ SA_LEN - sizeof ( sa_family_t ) ];
-};
+} __attribute__ (( may_alias ));
 
 #endif /* _GPXE_SOCKET_H */

src/include/gpxe/tcpip.h

 	 */
 	char pad[ sizeof ( struct sockaddr ) -
 		  ( sizeof ( sa_family_t ) + sizeof ( uint16_t ) ) ];
-};
+} __attribute__ (( may_alias ));
 
 /** 
  * A transport-layer protocol of the TCP/IP stack (eg. UDP, TCP, etc)

src/include/gpxe/udp.h

 #include <gpxe/if_ether.h>
 
 struct xfer_interface;
-struct sockaddr;
 
 /**
  * UDP constants