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

 # m68k specific configuration file

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORTS_WITH_CPU

 ##

 ## help The m68k architecture