Allow renewal of DHCP/BOOTP without setting the netif down

We had a meeting regarding the DHCP/BOOTP subsystem of lwip and conflicting requirements from the CIP-world and the SPC-world which we can only properly address by adding/enabling the feature to explicitly trigger a DHCP/BOOTP-Renew on an already active netif.

Since also some other defects were discovered with DHCP, please refer to the meeting notes, we decided that we would benefit from a full overhaul of the DHCP-integration along with adding this feature.

Thus, I will create a branch of lwIP for this ticket, fix what we have discussed to the best of my knowledge and add the new feature along the way. Target version for merging this back is still to be clarified.