Adding DCCP support

[onestay]

I would like to add DCCP support. It is currently only available on Linux. It would basically only involve exposing the Type, Protocol and some socket options listed here. Would this be something of interest?

Currently I'm proposing to just add something like this

    #[cfg(target_os = "linux")]
    /// Type corresponding to `SOCK_DCCP`
    /// 
    /// Used for the DCCP protocol
    pub const DCCP: Type = Type(sys::SOCK_DCCP);

and

#[cfg(target_os = "linux")]
    /// Protocol corresponding to `DCCP`
    pub const DCCP: Protocol = Protocol(sys::IPPROTO_DCCP);

In addition I'd propose adding the socket options prefixed with dccp_. So for example the socket option DCCP_SOCKOPT_SERVICE could be dccp_set_service() and dccp_service()

[Thomasdezeeuw]

Sounds good 👍

[Thomasdezeeuw]

@onestay can this be closed after #359 was merged?

[onestay]

Oh, yeah of course. Sorry, totally forgot.