Connection: Support IPv6 addresses

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
scripts/mosh: Let IO::Socket resolve the hostname

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
scripts/mosh: Add -6 option to connect with IPv6

Signed-off-by: Anders Kaseorg <andersk@mit.edu>