Release 3.12.0.

* `ObjectServer` should dispatch calls without destination since p2p connections typically don't have
* Only enable features of `nix` that we use.
* Support server-side cookie auth. We already supported client-side cookie-auth. zbus won't manage
  the cookies for you though, that's left to the bus implementation.
* Declare `ConnectionBuilder` as `must_use`. Instead of every builder method of it individually.
* Avoid a few string allocations in handshake code.
* Don't ignore errors from SID conversion to string.
* Add missing cfg on an import. Otherwise we get a warning when building for non-unix.
* Dependencies changed:
  * Home directory determination now split into a separate crate: `xdg-home`.
  * Drop `lazy_static`.
  * Drop now unneeded `dirs` dep.
  * Drop `nix` dep for non-unix (i-e Windows).