sys/socket: add UnixAddr abstract name getter

[lucab]

This introduces an as_abstract() getter to UnixAddr in order to
retrieve the name of an abstract unix socket.
This also adds tests around abstract addresses and clarify docs,
adding explicit semantics.

[lucab]

It looks like master is currently broken due to this libc change: rust-lang/libc#787.
/cc @vojtechkral

I also realized I forgot the relevant CHANGELOG entry, I'll add it on the next rebase once master is fixed. Reviews and comments still welcome in the meanwhile.

[vojtechkral]

Hi @lucab
To fix this, you probably just need to change #[repr(i32)] of the EventFilter to #[repr(u32)]. (OTOH if you believe my fix was incorrect or should not have been merged, please feel free to comment in the libc issue thread.)

[asomers]

Even though new_abstract and as_abstract compile fine everywhere, they won't work except on Linux. So they should probably be guarded with a #[cfg(target_os = "linux")]

[lucab]

PTAL on second iteration:

• rebased on a green master
• added CHANGELOG entries
• restricted abstract addresses methods to Linux only

@asomers I did as you suggested but I am not a fan of it, because UnixAddr is just a container for a typed address. I don't see why it should conditionally change depending on where you build. Perhaps a LinuxExt trait?

[asomers]

@lucab The #[cfg(target_os = "linux")] guard doesn't care about where you build, only where you target. And abstract sockets only work on Linux*. So it doesn't make sense to enable that code for any other OS. And I don't see what value a LinuxExt trait would add. Could you please explain?

• It would be worth checking if Android, which is considered a separate OS by Rust, supports them too.

[lucab]

@asomers I'm fine with your feedback, I don't want to keep bikeshedding this further.

I've double-checked Android docs and then added support for it as well, PTAL.

[asomers]

Thanks for your submission.

bors r+

[bors]

Build succeeded

• buildbot/nix-rust/nix amd64_fbsd11
• buildbot/nix-rust/nix i386_fbsd11
• continuous-integration/travis-ci/push