readline only handles SIGWINCH

readline only handles SIGWINCH

* acinclude.m4 (GUILE_READLINE): Check for rl_catch_signals and
  rl_catch_sigwinch.

* guile-readline/readline.c (scm_init_readline): If we can, turn off
  readline's signal handling, because we can do our own.
  (scm_readline): Use dynwinds to handle resetting readline's state on
  nonlocal exit, not catches.
  (unwind_readline): Rename from handle_error.