Re-init _Py_UnhandledKeyboardInterrupt before run. (GH-11963)

Explicitly reinitialize this every eval *just in case* someone is
calling into an embedded Python where they don't care about an uncaught
KeyboardInterrupt exception (why didn't they leave
`config.install_signal_handlers` set to `0`?!?) but then later call
`Py_Main()` itself (which *checks* this flag and dies with a signal after
its interpreter exits).  We don't want a previous embedded interpreter's
uncaught exception to trigger an unexplained signal exit from a future
`Py_Main()` based one.