Support as many abstract datagram unix domain socket operations as possible

[ntninja]

Motivation

Currently connecting/binding to the abstract namespace only works with stream Unix sockets.

Solution

This adds binding to abstract namespace names on datagram Unix sockets and returns error and documents limitations for other datagram socket OPs, as there is no support for specifying a SocketAddr for these in MIO currently unfortunately.

[ntninja]

This the test failure related to this? (Doesn’t look to me, at least.)

[ntninja]

“Fixed” CI by rebasing

[Darksonn]

I have a hard time understanding how and whether this changes behavior of Tokio.

Please add some tests that attempt to use these APIs with the abstract namespace, and set up your commits so that I can see the behavior before and after your change.

[ntninja]

@Darksonn: Added a test for using UnixDatagram::bind() with an appropriately formatted abstract socket path. It tests that this fails in the expected way on non-Linux as well. The usage of the standard library support for abstract datagram unix domain sockets to send a datagram to the bound socket is because it is impossible to implement support for sending on the UnixDatagram object without prior support in mio.