Allow to disable internal DHCP server
In https://gitlab.com/emulation-as-a-service/slirp-helper, we are trying to replace the (unmaintained) slirpvde with slirp-helper and a minimal wrapper to set up the data file descriptor. slirpvde allows to disable the internal DHCP server, which libslirp currently does not allow to (if I did not overlook an option).
This merge request disables the DHCP server if "0.0.0.0" is configured as DHCP start IP address, which seems reasonable. Currently, libslirp will happily assign (the invalid) "0.0.0.0" to clients if you specify it as DHCP start IP address. An alternative would be to introduce a new separate option to disable the DHCP server.