Now, that sub-interface setup is split by IP type, and separate checks
are performed for DHCP selection, it is possible to control DHCP on v4
an v6 sub-interfaces instantly. Add "dhcpv6" variable, akin to QMI
option, to control behaviour of DHCPv6 separately from IPv4 option,
which is required for some mobile operators.
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>