`copyEscapable` is a copy/paste of io.Copy with some added handling for
checking for the attach escape sequence.
This removes the copy/paste and uses `io.Copy` directly. To be able to
do this, it now implements an `io.Reader` which proxies to the main
reader but looks for the escape sequence.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Upstream-commit: 6380e3d939a5f992dc428febf02c25b0668d672c
Component: engine