interfaces: move peer `capabilities()` to the `ipeercapabilities` interface I'm not sure why this was on the `ipeercommands` interface. It appears to be because these interfaces started out as `_basewirecommands` to hold wire commands, back in 558f5b2ee10e. The capabilities interface wasn't split out until 98861a2298b5, when it pulled the capability related methods off of the `ipeerbase` interface. Perhaps it was an oversight to not look at the commands interface because, while this is a wire command, both `sshpeer` and `httppeer` now perform a handshake while instantiating the peer object, and cache a fixed list of capabilities in that object. Likewise, `localpeer` is given a fixed set of capabilities when instantiated. Back in 558f5b2ee10e, `httppeer` looks like it issued a wire command when this method was called, but `sshpeer` obtained and cached the capabilities when instantiated, and this method always returned a fixed value. There's a perfectly good interface with other capability related methods, and having it here makes it easier to implement the base `peer` mixin class.