net: Add support for v4-mapping-to-v6 sockets

This allows IPv4 and IPv6 share the same port space.
User can still control the behavior of the v4-mapping-to-v6
by using the IPV6_V6ONLY socket option at runtime.
Currently the IPv4 mapping to IPv6 is turned off by
default, and also the IPV6_V6ONLY is true by default which
means that IPv4 and IPv6 do not share the port space.
Only way to use v4-mapping-to-v6 is to enable the Kconfig
option and turn off the v6only socket option.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>