Add sysinfo::Groups implementation

[sisungo]

This PR tries to implement #1198. It contains:

• NEW PUBLIC API: sysinfo::Groups
• Structure Change: sysinfo::Group's internal structure is changed to have a GroupInner.
• Changes to users modules, in order to adapt the structure change.

When trying to implement the feature, I found on Windows, gid is always recognized as 0, but a SID is available. This PR didn't change Windows GID to SID as it may break public interface.

[GuillaumeGomez]

Any reason for this change? Gid is already generated based on the OS so what needed for a GroupInner type to be created?

[GuillaumeGomez]

That's the main reason why I didn't provide a Group API: Windows gid is pointless, making the API kinda useless and incoherent on this platform.

[GuillaumeGomez]

It's a great start, thanks! I have a few questions and also an annoying blocker: this new API is currently not useable as is on Windows (get_by_id doesn't work there, and it's an issue). So please remove these helpers, people have access to the data already with list and list_mut.

[sisungo]

getgrent, setgrent and endgrent are likely unavailable on Android?

[GuillaumeGomez]

It's available on android but:

#if __POSIX_VISIBLE >= 200112 || __XPG_VISIBLE
/* Android has thousands and thousands of ids to iterate through */
struct group* getgrent(void) __attribute__((warning("getgrent is inefficient on Android")));
void setgrent(void);
void endgrent(void);
int getgrgid_r(gid_t, struct group *, char *, size_t, struct group **);
int getgrnam_r(const char *, struct group *, char *, size_t, struct group **);
#endif

So I suppose the current approach is ok. Also, it's libc which might not have it, but it's available on android.

[GuillaumeGomez]

I checked a bit further and it's discouraged to use getgrent (as we can see in the code I copied) so I think it might be better to parse the file as you originally did (sorry).

About GroupInner, since it's exactly the same type for all platforms, here what I suggest: create the type in common.rs as pub(crate) and then implement GroupInner in unix, unknown and windows.

I see you also removed the helpers on Groups. Well done! 👍

So with this, I think it'll be complete. Sorry again for the back and forth. ^^'

[GuillaumeGomez]

Is not possible to declare GroupInner here and to implement in the sub modules?

[GuillaumeGomez]

Code implementation looks good! Just one thing missing: a test. Can you add a test checking that Groups is not empty on supported platforms please? Something like:

#[test]
fn test_groups() {
    if !crate::IS_SUPPORTED_SYSTEM {
        return;
    }
    assert!(!Groups::new_with_refresh_list().is_empty());
}

[sisungo]

Unit tests added.

[GuillaumeGomez]

Great work, thanks a lot!