add linux abstract socket support

Linux has had abstract socket support for a very long time.

They solve problems like MDEV-5479, where its hard to tell if a residual file system socket is in use, and not possibly to atomically take over the bind. Abstract socket would solve part of this by ensuring that a terminated program removes it abstract socket (the kernel does it automatically).

A lot of effort has been spent writing init scripts that cleanup filesystem sockets too.