libedit-preferred by default

If both libedit and libreadline are available, we currently default to
linking against libreadline, unless the "--with-libedit-preferred"
configure option is used. Why do we default to preferring GNU readline
over libedit?

libedit is clearly preferable for license reasons. You could make the
argument that libedit is perhaps less stable than readline (although I'm
not aware of any outstanding issues with it), but (a) we need to support
libedit anyway, due to Darwin and NetBSD (b) if there are any issues
with libedit, users can always specify "--without-libedit-preferred" (c)
those users with both libraries installed are presumably at least
somewhat satisfied with libedit.

-Neil