WebServer: if ipv6 is available, try to start two MHD instances,
one on ipv6, then one on ipv4. On dual-stack systems where listening
on ipv6 also accepts ipv4 connections (at least OSX), only
one thread will be used and serve both address families.
On others, two threads will be started.
JSONRPCServer, AirPlayServer: listen on ipv6 if available.
Try to open an ipv6 socket and disable IPV6_V6ONLY to make
it dual stack. If we can't get a socket because ipv6 is not
supported at runtime (i.e. ipv6.ko not loaded), fallback to
ipv4.
Merge pull request #2718 from simonvetter/xbmcv6

JSONRPCServer, AirPlayServer, WebServer: listen on ipv6 if available, else fall back to ipv4