uart_pty: Added a debug terminal

You can now launch an xterm with picocom by setting environment
SIMAVR_UART_XTERM. This could (will) be improved.

Also, the uart_pty symlinks the pty pathname to /tmp/simavr-uart-X
so you can rely on a constant pathname.

Signed-off-by: Michel Pollet <buserror@gmail.com>