SocketUtil: explicitly set V6ONLY socket option

[nmeum]

Currently, MPD emits the following warning on Linux when
net.ipv6.bindv6only is set to the default value zero:

exception: bind to '0.0.0.0:6600' failed (continuing anyway, because binding to '[::]:6600' succeeded): Failed to bind socket: Address already in use

This is a known issues (#673, #557, …) caused by the fact that MPD, by
default, creates separate sockets for AF_INET and AF_INET6. With
this architecture it is not necessary to use the AF_INET6 socket for
both IPv6 and IPv4. For this reason, explicitly enable the V6ONLY
socket option for AF_INET6 sockets, thereby preventing the error
causing the warning to be emitted.

[MaxKellermann]

I thought of doing this a few times, but didn't because I didn't want to override the admin's global setting for this. By default, MPD should be a good citizen and should follow global configuration.
And don't forget: your proposed code change breaks configurations where admins have disabled "v6only" globally and made MPD bind to "::"; with your change, they can't use IPv4 anymore.
Do you have a good justification for this breaking change?

[nmeum]

And don't forget: your proposed code change breaks configurations where admins have disabled "v6only" globally and made MPD bind to "::"; with your change, they can't use IPv4 anymore.

I think the amount of people doing that is close to zero. Besides, if you bind a socket to a specific IPv6 address through a software configuration option I would not expect the software to create an IPv4 socket as well. Also keep in mind that this only works for in6addr_any. If you bind explicitly to some other IPv6 address it will only create an IPv6 socket, even on current git head. If the intention of the admin is to bind MPD to both IPv6 and IPv4 he can simply use bind_to_address "any" instead of bind_to_address "::" in the first place.

I thought of doing this a few times, but didn't because I didn't want to override the admin's global setting for this. By default, MPD should be a good citizen and should follow global configuration.

I don't think that this is a popular interpretation of net.ipv6.bindv6only. In my experience, most software sets this socket option explicitly, either creating a single IPv6 socket and disabling V6ONLY or creating two sockets and enabling V6ONLY.

[neheb]

This needs to be rebased to pass Travis.

[nmeum]

Rebased and fixed the build on Mac OS.