mosh unusable with IPv6 in Cygwin

mosh’s IPv6 support has been broken in Cygwin for a while.

Network exception: setsockopt: Invalid argument
[mosh is exiting.]

If only the error message had reported source file and line number.

Listen up, boys and girls, hackers and hookers, do error messages properly. If not, I’ll magically materialize next to you, ensuring you do this right.