Only do the IPV6_V6ONLY setsockopt(3) stuff if that option is available.

Should fix #177, in which compilation failed on older Solaris systems
that didn't have it.  This is a different approach than a patch
suggested in that issue.

Weakly regression-tested on other platforms (test this by specifying
-6, -4, or neither to the server when binding to the wildcard address,
and seeing if a client can connect with various of -6, -4, or neither).

(cherry picked from commit 147d3369a0a9f448875088915e74a284a46436ab)
Signed-off-by: Bruce A. Mah <bmah@es.net>

Conflicts:
	src/iperf_sctp.c
Don't try to do IPV6_V6ONLY on OpenBSD either.

setsockopt(3) returns an error if passing 0 to this option (which
we do if no address family is specified when we bind to the wildcard
address, say by invoking "iperf3 -s" with no other options).  This
is because OpenBSD explicitly does not support IPv4-mapped addresses,
so even though the IPV6_V6ONLY socket options exists, it only works
with a non-zero argument.

Fixes #196.

(cherry picked from commit bef5ef87f58a4953a7165d3a5242f2a49cec32cf)
Signed-off-by: Bruce A. Mah <bmah@es.net>

Conflicts:
	src/iperf_sctp.c
Update release notes, somewhat optimistically plug in a release date.
Bump version number.
Regen.
Remove a "known issue" that was fixed.

(cherry picked from commit 263449b1d28f023e1ff2818a04e1e2b00693e78c)
Signed-off-by: Bruce A. Mah <bmah@es.net>