test: add NetBSD support to test-udp-ipv6.c
test: add NetBSD support to test-udp-ipv6.c

NetBSD and FreeBSD can share the can_ipv6_ipv4_dual() function as
is. This commit also corrects the function definition to be
compliant with C standards. This fixes a warning on
NetBSD 8.99.2 with GCC 5.4.0.

PR-URL: https://github.com/libuv/libuv/pull/1535
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso