feat: implement abstract UNIX domain socket names

[sivizius]

1. Harmonise spelling of »UNIX domain socket« as per https://man7.org/linux/man-pages/man7/unix.7.html
2. Allow abstract names in various places, prefixed with @, which is used by systemd, https://man7.org/linux/man-pages/man8/ss.8.html, etc.

[zgttotev]

Sorry if this is the wrong place but I was looking for X11 forwarding support to a local abstract socket per the patch in Fedora: https://src.fedoraproject.org/rpms/openssh/blob/f43/f/0007-openssh-7.2p2-x11.patch
This PR seems to add support for an @ prefixed path to connect_local_xsocket_path but its caller connect_local_xsocket never tries an abstract socket (like it does in the Fedora patch). Would this be something you'd be willing to add?

[zgttotev]

Appears there is prior art on this subject I'm just uncovering. https://tstarling.com/blog/2016/06/x11-security-isolation/ points to Bug 1789 which attempted to add this support but was unsuccessful due to security concerns.