net: Remove s_addr/s6_addr defines from in_addr/in6_addr

Replace #defines for s_addr/s6_addr etc. in in_addr/in6_addr structures
within net_ip.h with fixed fileds inside an anonymous union. This
prevents intrusive behaviour of net_ip.h, which expands every occurence
of s_addr/s6_addr with it's own define, even in other, non-related
structures.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>