DEBUG/debugpline() fixup

DEBUG/debugpline() fixup

 * Replace variadic debugpline() with fixed argument debugpline0(str),
   debugpline1(fmt,arg), and so on so that C99 support isn't required;
 * showdebug() becomes a function rather than a macro and handles a
   bit more;
 * two debugpline() calls in light.c have been changed to impossible();
 * DEBUGFILES macro (in sys.c) can substitute for SYSCF's DEBUGFILES
   setting in !SYSCF configuration (I hope that's temporary).