Use \r\n instead of \n in terminal output

[mutantmonkey]

The default in golang.org/x/crypto/ssh/terminal was changed to set
OPOST, which disables output postprocessing like ONLCR, which previously
handled this for us. As a result, a carriage return was not being added
to newlines in terminal output, which lead to weird display issues.

Fixes #124.

[unkaktus]

IMHO it should be fixed upstream (golang/go#17364) by making a method of setting OPOST flag back. This will require just a few lines of code when initializing a terminal instead of doing s/\\n/\\r\\n/g.
Even if you fix all write() calls here, the lack of OPOST flag also breaks everything else.

[agl]

Should be fixed by https://go-review.googlesource.com/#/c/33902/.