Allow DA address to be specified as FQDN

It would be very helpful, particularly in sandbox situations, to specify the Directory Authority by FQDN hostname instead of by IP address. This would allow us to defer picking an actual IP address until the simulation is started, and even to use some "in-game" DNS facility to figure out the actual address after the simulation is launched.

Right now, specifying a FQDN for the "DirAuthority" config file entry even partially works already: if the FQDN happens to start with a digit, it is correctly resolved internally using available DNS resolver infrastructure :)

The first attached patch makes that work in all cases (even when the FQDN hostname does not begin with a digit).

The second attached patch allows FQDNs to be inserted into DA certs created using tor-gencert, and correspondingly resolved when a client parses the downloaded DA certificate.

I realize there is ongoing work to refactor parsing the DA config entry (ticket #17224 (moved)), so please consider this patch set either independently on its own merits or as part of that larger effort. In the first case, I'd be happy to redo and resubmit the patches based on review/feedback.

Trac:
Username: somlo