Implement the ssh(1) escaping rules.

The first rule of ssh(1) escaping is that there is no escaping.

This doesn't undo any of my recent security fixes because they're all
calling escape_arg themselves.

This fixes "adb shell rm /data/dalvik-cache/arm/*".

Also remove do_cmd which caused concern during code review.

Bug: http://b/20564385
Change-Id: I4588fd949d51e2a50cff47ea171ed2d75f402d0d