Pull request for some Get/Set set-UID/set-GID/stick-bit attributes + UID/GID context of the userland processus invoking syscall

[folays]

Hello, what do you think of this PR ?

It has been tested on Mac & Linux (on Linux where it worked in production for at least 1 year+)

Those are needed to be able to 100% pass a POSIX test suite at least on Linux (I used https://github.com/pjd/pjdfstest)

This PR adds ability :

• to "receive" requests to change set-UID/set-GID/sticky-bit (chmod u+s / g+s / +t) in .SetInodeAttributes()
• to figure out the "context" (UID/GID) of the user land processus invoking a syscall (I think I probably needed this one to have newly created inode be POSIX-compliant (have the correct uid:gid and/or maybe some stick-bit group folder stuff, I don't remember)

Mac : I didn't verified the correctness or meaningfulness of those fields, but at least it doesn't prevent anything from working. Linux : It works as expected. Windows : Didn't tested at all.

[stapelberg]

Thanks!