Don’t stuff keyboard input uselessly

Also, document stuffing better.
* src/keyboard.c (stuff_buffered_input):
Give up on stuffing if it fails.
* src/sysdep.c (stuff_char): Return failure indication.

admin/notes/multi-tty

@@ -504,6 +504,8 @@ THINGS TO DO
 ** flow-ctrl.el must be updated.
 
 ** Fix stuff_char for multi-tty.  Doesn't seem to be of high priority.
+   For security reasons stuff_char does not work by default on many
+   platforms, which makes such a fix even lower priority.
 
 DIARY OF CHANGES
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