Added lean config property com.openexchange.ipcheck.mode, which specifies the IP check mechanisms to apply.

Known values are: none, strict and countrycode
- none implies that no IP check takes place at all
- strict implies that IP addresses are checked for equality
- countrycode requires that package open-xchange-geoip is installed and performs a plausibility check against IP addresses' country codes; so check fails if country codes differ

Note: The com.openexchange.IPCheck property still has precedence over this property;
i.e. if com.openexchange.IPCheck is set to true, strict IP check is enabled.

Default is empty