Use an FDroid specific NSD service type, based on SSL preference.

NSD service types follow a convention _protocol._transport. Previously
Kerplapp was advertising an _http._tcp service generically. It makes
more sense to advertise a _fdroid._tcp service so the FDroid client can
run a discovery that it knows won't return generic non-FDroid repo HTTP
servers.

Further, we should advertise _fdroid._tcp when the use_ssl preference is
disabled and _fdroids._tcp when it is enabled to indicate to the
discovering peer whether we are advertising an HTTP or an HTTPS FDroid
repo.