Use HEAD before attempting GET for susidns subscriptions

[Reportage]

SusiDNS should check the timestamp of subscription urls via a HEAD request and only perform a GET if the timestamp is newer than the existing local subscription, to save bandwidth and server load.

[zzz]

Not necessary, we already do something even better.
SusiDNS+eepget support standard HTTP conditional headers Etag and If-Modified-Since. Susidns stores and returns the values from the last fetch. Compliant web servers will return 304 if not modified. No separate HEAD is required.