Implement IPv6 support in drawterm.

This change has been reviewed by Russ Cox
and merged in his drawterm repository.

See the changesets:

https://bitbucket.org/rsc/drawterm/commits/a43b5147ae918478898b9b0841c9977f3bdb9a09
https://bitbucket.org/rsc/drawterm/commits/4a8b3bbeab0bb2c637d0055e46d86c8aac949e70