Implement SIGWINCH handler so that Edwin console screens can resize

Implement SIGWINCH handler so that Edwin console screens can resize
themselves if their enclosing terminals are resized.  This is
accomplished by:

  - allocating a new interrupt, INT_Global_3, to mean that the
    console was resized;
  - installing a SIGWINCH handler to request an INT_Global_3
    interrupt;
  - introducing a new event distributor, EVENT:CONSOLE-RESIZE, which
    the run-time system's INT_Global_3 handler invokes; and
  - adding edwin/tterm.scm's RESIZE-SCREEN as an event receiver to
    EVENT:CONSOLE-RESIZE whenever an Edwin console screen is entered,
    and removing it whenever a console screen is exited.