(continuation-prompt): Renamed from "prompt2".
(make-readline-port, readline, set-readline-prompt!): Reflect above renamings.
(activate-readline): Rename locals "read-hook" and "prompt" to
"repl-read-hook" and "repl-prompt", to disambiguate them from
globals. Save and restore the new-input- and continuation-
prompts around the REPL read call.
Added LIBGUILEREADLINE_MAJOR variable for inclusion in the name of
the shared library.
* configure.in: AC_SUBST it.
* Makefile.am: Substitute it into name of library.
* ice-9/readline.scm: Use new name with load-extension.
