Revert "Sync readline/ to version 7.0 alpha"

This reverts commit b558ff043d.
This reverts commit 4a11f20659.

The initial import commit failed to retain local changes made to
readline's configure.in (and the commit message erroneously stated that
there were no local changes that needed to be reapplied).  Also the
import caused a couple of build errors and a scattering of testsuite
regressions throughout many arches.  It's probably better to start over
with this import, hopefully more carefully next time.

readline/rlstdc.h

@@ -42,16 +42,4 @@
 #  endif
 #endif
 
-/* Moved from config.h.in because readline.h:rl_message depends on these
-   defines. */
-#if defined (__STDC__) && defined (HAVE_STDARG_H)
-#  define PREFER_STDARG
-#  define USE_VARARGS
-#else
-#  if defined (HAVE_VARARGS_H)
-#    define PREFER_VARARGS
-#    define USE_VARARGS
-#  endif
-#endif
-
 #endif /* !_RL_STDC_H_ */