std: Expose a mode accessor for Permissions on unix

[alexcrichton]

Currently we have a set_mode mutator, so this just adds the pairing of a
mode accessor to read the value.

Closes #22738

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

r? @aturon (or others who feel comfortable)

[aturon]

@bors: r+ 537d694

[aturon]

Note: I am not necessarily happy with this API in the long run!

[bors]

⌛ Testing commit 537d694 with merge 8555419...

[bors]

💔 Test failed - auto-win-32-nopt-t

[jondot]

@alexcrichton @aturon coming from the future to haunt you, well, it turns out that reading mode gives out a 16-bit format (includes the file type). It probably relays what ever value the OS has given directly.

However reading after immediately setting the classic 9 bit format (755 for example), gives out the same 9 bit format value, which is different from when we read directly without setting (it gives 16 bit value). It probably relays what ever value Rust cached directly which results in the same 9 bit value which we just set.

So this will fail on the second assert:

    let f = File::create("foo.bar")?;
    let metadata = f.metadata()?;
    let mut permissions = metadata.permissions();
    permissions.set_mode(0o755);
    assert_eq!(permissions.mode(), 0o755);
    fs::set_permissions("foo.bar", permissions)?;

    let f = File::open("foo.bar")?;
    let metadata = f.metadata()?;
    let permissions = metadata.permissions();
    assert_eq!(permissions.mode(), 0o755);   // we are actually getting 0o100755 from the OS here

while the data is correct on the filesystem, this is a surprising behavior in the API.