ptyfwd: optionally, change ANSI background color of forwarded terminals

ptyfwd: optionally, change ANSI background color of forwarded terminals

As the bytes flow through our terminal forwarder we can color the
background of the terminal with a color of our choices, if that's
desired.

This will later allow us to color the background of the uid0 tool when
running as root with a slightly alarming red color.

This does two things:

1. When an ANSI sequence is seen that resets the background color, it is
   extended to immediately set the color to our choice.
2. When a newline is seen it is immeidately extended to set the
   background color again and clear the current line till the end.

Net effect: all lines written while we forward the ttys will be shown
with the background color of choice.