systemd unit file: set up /var/run/tor as writable for the Tor service.

For some strange reason, this was not needed with systemd v208.
But it's needed with systemd v215 on current Debian sid, and entirely
makes sense.

contrib/dist/tor.service.in

@@ -22,6 +22,7 @@ InaccessibleDirectories = /home
 ReadOnlyDirectories = /
 ReadWriteDirectories = @LOCALSTATEDIR@/lib/tor
 ReadWriteDirectories = @LOCALSTATEDIR@/log/tor
+ReadWriteDirectories = @LOCALSTATEDIR@/run/tor
 NoNewPrivileges = yes
 
 [Install]