input.__promptme is no valid indicator that run_shell should be left.
It should be left on executing the 'exit' builtin which is indicated
by a return code < 0 from parse_stream_outer(). Track this with an extra
variable and use it as a condition to return from an interactive shell.

This fixes the weird behaviour that hush exits (and the user finds itself
in the menu) when a syntax error occured.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>