scripts/functions: fix endless loop in debug-shell with IO redirection

CT_DEBUG_INTERACTIVE is disabled when stdin, stdout or
stderr are redirected, but the check is only done at
the start of the build and doesn't catch when individual
build commands use redirection. When stdin is redirected
it will cause the debug shell to exit immediately, causing
and endless loop. Thus, save the stdin/our/err file handles
and restore them before invoking the debug shell.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
Message-Id: <20121030102225.GA8303@sig21.net>
Patchwork-Id: 195409

 diff -durN gcc-4.4.0.orig/libgomp/configure gcc-4.4.0/libgomp/configure

 --- gcc-4.4.0.orig/libgomp/configure	2009-04-21 11:08:08.000000000 +0200

 +++ gcc-4.4.0/libgomp/configure	2009-05-27 21:38:19.000000000 +0200

 @@ -3334,7 +3334,7 @@

  # Add -Wall -Werror if we are using GCC.

  if test "x$GCC" = "xyes"; then

 -  XCFLAGS="$XCFLAGS -Wall -Werror"

 +  XCFLAGS="$XCFLAGS -Wall"

  fi

  # Find other programs we need.