Allow DNS names in Directory Service client config

[lakeman]

In overlay_address.c load_subscriber_address(), refactor to allow asynchronous resolution of configured domain names. This would avoid the problem of hard-configured IP addresses, which may change.

[lakeman]

Or Serval Mesh could store a domain name in its preferences, then resolve and set the IP address...

[lakeman]

DNS resolution has been implemented as a synchronous process. For the moment, this is a rare enough occurrence that we shouldn't need to worry about the timeout issue.

[quixotique]

Warrants at least a TODO comment in the source at the point where it blocks. Possibly also warrants a new issue (referencing this one).

[lakeman]

overlay_link.c, resolve_name() already has a brief TODO.