net: support for bridged networking on Mac OS X

tun tap can be implemented on Mac OS X using
http://tuntaposx.sourceforge.net

It behaves in the same way as FreeBSD/OpenBSD implementations, but Qemu
needs a patch to use the OpenBS/FreeBSD code.

As per the patch listed in this forum thread:
http://forum.gns3.net/post17679.html#p17679

And also as used in the MacPorts installation:
https://trac.macports.org/browser/trunk/dports/emulators/qemu/files/patch-net-tap-interface.diffSigned-off-by: NAlasdair McLeay <alasdair.mcleay@me.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

net/tap-bsd.c

@@ -44,7 +44,8 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
     struct stat s;
 #endif
 
-#if defined(__FreeBSD__) || defined(__FreeBSD_kernel__) || defined(__OpenBSD__)
+#if defined(__FreeBSD__) || defined(__FreeBSD_kernel__) || \
+    defined(__OpenBSD__) || defined(__APPLE__)
     /* if no ifname is given, always start the search from tap0/tun0. */
     int i;
     char dname[100];