This diff adds to miniooni support for using the torsf tunnel. Such a
tunnel consists of a snowflake pluggable transport in front of a custom
instance of tor and requires tor to be installed.
The usage is like:
```
./miniooni --tunnel=torsf [...]
```
The default snowflake rendezvous method is "domain_fronting". You can
select the AMP cache instead using "amp":
```
./miniooni --snowflake-rendezvous=amp --tunnel=torsf [...]
```
Part of https://github.com/ooni/probe/issues/1955
