onion-grater race condition: move OnionShare to its netns

In a separate issue, @segfault pointed me at https://seclists.org/oss-sec/2020/q4/91 CVE-2020-25653: UNIX Doman Socket Peer PID Retrieved via SO_PEERCRED is Subject to Race Condition

This mechanism is similar to how onion-grater works, so it's probably vulnerable, too.

At the moment only OnionShare remains to be fixed.

As anonym noted on 42fd3adf, "we really have to clean up" Onion Grater once this is done. See that commit for details.