Peer credentials on Unix socket

[zeenix]

This is also something that's needed for D-Bus use case but unlike #623, this is needed for modern systems/software as well, not just backwards incompatibility with legacy systems so much higher priority IMO than that. This is already supported in tokio but not yet in std.

[zeenix]

@yoshuawuyts For this one, we could just use the same API as tokio, no?

[yoshuawuyts]

@zeenix probably! -- I see Alex is the one that signed off on the API in that thread, which adds some level of trust it's been thought through correctly. A PR to add this (as unstable) would be welcome!