Clean up the unsightly mess around the readline header files. We now

Clean up the unsightly mess around the readline header files.  We now
always:

- #undef HAVE_CONFIG_H (because otherwise chardefs.h tries to include
  strings.h)

- #include readline.h and history.h

and we never declare any readline function prototypes ourselves.

This makes it compile with readline 4.2, albeit with a few warnings.
Some of the remaining warnings are about completion_matches(), which
is renamed to rl_completion_matches().

I've tested it with various other versions, from 2.0 up, and they all
seem to work (some with warnings) -- but only on Red Hat Linux 6.2.

Fixing the warnings for readline 4.2 would break compatibility with
3.0 (and maybe even earlier versions), and readline doesn't seem to
have a way to test for its version at compile time, so I'd rather
leave the warnings in than break compilation with older versions.