systemd unit file: only allow tor to write to /var/lib/tor and /var/log/tor (#12751).

The rest of the filesystem is accessible for reading only. Still, quoting
systemd.exec(5):

  Note that restricting access with these options does not extend to submounts
  of a directory that are created later on.
systemd unit file: ensures that the process and all its children can never gain
new privileges (#12939).
Add changes file for #12939.
Add changes file for #12751.
Merge remote-tracking branch 'intrigeri/bug12751-systemd-filesystem-sandbox'
Merge remote-tracking branch 'intrigeri/bug12939-systemd-no-new-privileges'

Conflicts:
	contrib/dist/tor.service.in