slirp: Factorizing tcpiphdr structure with an union

This patch factorizes the tcpiphdr structure to put the IPv4 fields in
an union, for addition of version 6 in further patch.
Using some macros, retrocompatibility of the existing code is assured.

This patch also fixes the SLIRP_MSIZE and margin computation in various
functions, and makes them compatible with the new tcpiphdr structure,
whose size will be bigger than sizeof(struct tcphdr) + sizeof(struct ip)

Signed-off-by: Guillaume Subiron <maethor@subiron.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
diff --git a/slirp/socket.c b/slirp/socket.c
index 55150f5..8df9252 100644
--- a/slirp/socket.c
+++ b/slirp/socket.c
@@ -483,7 +483,18 @@
 	  if (!m) {
 	      return;
 	  }
-	  m->m_data += IF_MAXLINKHDR;
+	  switch (so->so_ffamily) {
+	  case AF_INET:
+	      m->m_data += IF_MAXLINKHDR + sizeof(struct udpiphdr);
+	      break;
+	  case AF_INET6:
+	      m->m_data += IF_MAXLINKHDR + sizeof(struct ip6)
+	                                 + sizeof(struct udphdr);
+	      break;
+	  default:
+	      g_assert_not_reached();
+	      break;
+	  }
 
 	  /*
 	   * XXX Shouldn't FIONREAD packets destined for port 53,