Add support for legacy ifupdown1 interfaces stanzas

[Maximilian Wilhelm]

ifupdown1 historically required multiple IPs / address families to be configured in separate interface stanzas which might have an address / netmask pair each; this might have looked like this:

iface eth0 inet static
    address 192.2.0.23
    netmask 255.255.255.0
    gateway 192.2.0.1

iface eth0 inet6 static
    address 2001:db8::23
    netmask 64
    gatetway 2001:db8::1

Today ifupdown1 supports CIDR notation so recent setups might not use a separate netmask anymore but there will be a lot of setups out there which use split interface stanzas and some might still use address / netmask. We should be able to correctly parse and verify those setups, maybe complain about missing netmask per stanza and inernally convert the config to CIDR notation.