Convert ^G back to 7 when sent.

This technically fixes issue 137 which specifically asked for
"backslash", but backspace (^H) and tab (^I) along with CR (^M)
still do a different thing than they do on Atari keyboards.

Further, CTRL-Q and CTRL-S are used by SyncTERM for Quit and Menu
respectively, so someone expecting all the Atari "stuff" to work
will still be disappointed.

This is a general problem with non-PC emulation (ie: Prestel, Atari,
and Commodore) that really needs some kind of general solution.