FER - valid keys

When FER is set, only certain keys are valid.  The ones I know of are
arrow keys, TAB, BACKTAB, BACKSPACE, ENTER, FIELD EXIT, FIELD +.  (This
was tested on a 3487.)  The odd thing about the "real" terminal is left
arrow which  doesn't move the cursor, all other arrow keys do.  I guess
it moves left back into the field.

In tn5250, RESET, left arrow, TAB, BACKTAB, BACKSPACE and FIELD EXIT
clear the FER.  There is something wrong with FIELD + (don't know if
it's the last patches from CVS that I got today or if it was like
that).  From display.c I think it was intended that FIELD+ and FIELD-
reset the FER.

I added K_ENTER to my display.c and it seems to work well.  I don't know
how to fix up arrow, right arrow, down arrow, FIELD+, FIELD- (if FIELD-
should really work - it doesn't on my "real" terminal).

--- display.c.old   Wed Dec  6 14:30:38 2000
+++ display.c   Mon Jan 22 16:26:54 2001
@@ -900,6 +900,7 @@
       case K_TAB:
       case K_BACKTAB:
       case K_RESET:
+      case K_ENTER:
         pre_FER_clear = 1;
         break;
+---
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+---