Add official setpriv-wrapper.sh which implements gosu in pure POSIX shell via setpriv

[tianon]

This promotes setpriv to the official (coveted) "top" spot in the list of Alternatives.

[tianon]

I think "fragile" is probably the best description of this code. It definitely does The Thing, but probably not the best answer for an actual "load bearing" component of another system, so perhaps I should leave this as-is and only include the README shuffling? Perhaps even with a reference to this PR for users who really want to dig themselves a hole? 🤔

[grooverdan]

FWIW SUSE folks have gone the setpriv approach https://github.com/SUSE/BCI-dockerfile-generator/blob/main/src/bci_build/package/mariadb.py#L20-L31

[tianon]

I don't love the implementation, but it's fine -- however, I really find it offputting that they call it gosu and think they should give it a different name. It is not even "gosu compatible" as written (for that, they'd need to use something like the script I've got in this PR, but even then shouldn't be called "gosu"). I've got a pretty extensive test suite of the edge cases of the gosu/Docker/libcontainer/runc "users" parsing here that I've successfully run the script in this PR through.

Edit: dug in to find the place it was added and raised this with those folks in SUSE/BCI-dockerfile-generator#963 (comment)

Edit 2: now in SUSE/BCI-dockerfile-generator#2070