Implement DNS-based DAV service discovery

Currently we do the following in dav/mod.rs (after merging #747):

• if the URL configured by the user has a path != "/", use it directly as homeset-URL (interpreting it as manual override)
• otherwise attempt to call GET https://{host}:{port}/.well-known/caldav (in get_well_known_url) and hope we will get redirected to the right URL
• if we get an error from the server, use the configured URL as base URL again

It would be nice if we got DNS-based discovery into our heuristics as well (see http://sabre.io/dav/service-discovery/). The biggest question is when we should do that. Two ideas come to mind:

Just add it to the chain

• Attempt to use configured URI directly, if it works, return
• Attempt well-known-based discovery, if it works, return
• Attempt DNS-based discovery, if it works, return

URL-less login

Or perhaps only attempt DNS-based discovery if the user didn't specify a URL and their username is an email address?

So this will trigger the current heuristics:

url = https://icloud.com
username = jeanne.doe@icloud.com
password = ...

But this will trigger DNS-based discovery:

username = jeanne.doe@icloud.com
password = ...

Start in dav/mod.rs, especially get_well_known_url and the discovery flow described after #747. Review the linked service-discovery reference and decide whether DNS lookup belongs in the fallback chain or only in URL-less email login. Done means the chosen discovery behavior and its ordering are implemented and verified.