Fix tarantool -e "os.exit()" hang

After the patch which made os.exit() execute on_shutdown triggers
(see commit 6dc4c8d7) we relied
on on_shutdown triggers to break the ev_loop and exit tarantool.
Hovewer, there is an auxiliary event loop which is run in
tarantool_lua_run_script() to reschedule the fiber executing chunks
of code passed by -e option and executing interactive mode.
This event loop is started only to execute interactive mode, and
doesn't exist during execution of -e chunks. Make sure we don't start
it if os.exit() was already executed in one of the chunks.

Closes #3966