Allow the SITL scripts to run on platforms without gnome-terminal

Introduce a new variable $APMTERM that specfies the terminal window
to spawn.  Defaults to xterm as the most commonly available option.

This changes the look and feel of the non-interactive terminals only so
most people won't notice a difference, but of course old behaviour can be
restored with

APMTERM=gnome-terminal sim_ardu...

or a bashrc export.