Partially fix signal (control-C) handling under mingw32.
The interpreter can now be interrupted and successfully returns to
the Hugs prompt, but only if Hugs was started from cmd.exe and not
by bash (!).  This is a known bogon with Cygwin, according to
Sigbjorn.  I still can't get Hugs to ignore other control-C's;
instead it acts as if it was asked to exit.  Bizarre.