include/ssh: Make sure handles don't contain line breaks

include/ssh: Make sure handles don't contain line breaks

Handles returned by ssh_tunnel_open() and ssh_proxy_open() are base64
encoded using the base64 commandline tool. However, base64 inserts line
breaks into long outputs, causing line breaks to be inserted into long
handles, and the returned handles to become unusable.
This commit changes the parameters passed tp base64 so that no line
breaks are inserted into the handles.