Add support for "keyboard" input over the serial port.

Revise documentation accordingly.
Revise reference solution accordingly.
Change real Return key to produce \r, to match what is received on
serial port.
Update shell example program to expect \r at end of file.
Modify testing makefiles to supply /dev/null as input.
Add squish-pty help program to deal with Bochs,
and modify "pintos" to use it.