- Add support for parsing IPv6 addresses out of data received via webcheck ip url.

- Configurable user-agent with version string placeholder (%V?)

- HTTP proxy authentication

- Add curl HTTP headers to disable proxy caching

- On too many update errors: Configurable block period and error threshold counter

- Log alternative server name on successful/failed update

- Clear external warning on successful update

- Configurable CA certificate handling

- Determine hostname TTL via DNS and remove option from config file

Nice to have:
- webcheck mode: Cache last known IP

  We only determine the current IP every "webcheck_interval" to prevent server trashing.

  If a service can't be updated right now (because the update interval forces an idle period),
  it will only be updated after the next successful webcheck IP determination.
  We could cache the last known IP and update the service as soon as
  the service's update interval permits.

- Move curl_* handling to own class/use HTTPHelper in ip_addr_helper

- Add option to send out updates f.e. once a week as a kind of keepalive message.