top: add more robustness to logic for reading a rcfile

top: add more robustness to logic for reading a rcfile

At this point I don't know if the Rc.id ever carried a
value of 'b', 'c', 'd' or 'e' in a public release. But
I do know that those values ought not to be recognized
in any configuration file. Now they won't be for sure.

[ the net result is that now a user with such a file ]
[ will see the 'incompatible rcfile' message instead ]
[ of that misleading 'window entry corrupt' message. ]

Signed-off-by: Jim Warner <james.warner@comcast.net>