net: Add IPv6 source address selection support

Add support for IPv6 Source Address Selection defined in RFC 5014.
This commit does not add any address selection support to getaddrinfo()
as described in the RFC. The AI_EXTFLAGS symbol and also the ai_eflags
to hints is added but they are not used by the code.
The public/temporary address selection is only done internally by
the net_if_ipv6_select_src_addr_hint() according to flags set by
setsockopt().

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