Use separate debconf questions for IPv4 and IPv6 netmask and gateway

Pascal Hambourgrequested to merge
pham/netcfg:static2 into master

netcfg/get_netmask and netcfg/get_gateway descriptions mention only IPv4 form. Invalid netmask and gateway error messages should be different for IPv4 and IPv6.

So add new dialogs for IPv4 and IPv6 and use them for netmask and gateway depending on the expected address family.

Note: This MR conflicts with MR !14 (merged) and must be updated after MR !14 (merged) is merged.

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