[bc] readline support broken

[Golubev I. N.]

> I think this belongs on bug-bash.

NAK, I really mean it: bc tries to support readline, and does that in
a broken way.  Readline itself needs no modification wrt to it, dozens
other packages link with it normally, and bc also should, and I
explain why it fails now.

Certainly there can be some readline add-on to facilitate readline
clients' configuration.