When to add flags to network status vs adding new items

[Roger Dingledine]

This is mainly for Nick, but maybe we'll get back into the habit of
making design discussions more public. :)

I'm in the process of implementing PreferTunnelledDirConns, which makes
you pick a directory server that supports BEGIN_DIR cells if one is
available. But the routerstatus_t or local_routerstatus_t structs don't
say what version the server is running, so I can't know. (And I can't
just fetch all the descriptors to learn the versions, because the whole
point is to fetch them using BEGIN_DIR when possible.)

One option is to add another flag to the status lines:
supports_dir_tunnels or the like, which is set when we believe the server
is running Tor 0.1.2.2-alpha or later.

Another option is to extend the router line in the status list to include
the version we think it's running, and then clients can make this decision
(and future decisions) themselves.

Both of these would be fine with me in this case. What's the right habit
down the road? Should we add new flags whenever there's a new capability,
or should we reserve flags for situations where the judgement is more
complex than a call to tor_version_as_new_as()?

--Roger