main: run an event loop for on_shutdown triggers

When Tarantool is stopped by Ctrl+D or by reaching the end of the
script, run_script_f() breaks the event loop, then tarantool_exit()
is called from main(), however the fibers that execute on_shutdown
triggers can not be longer scheduled, because the event loop is
already stopped. Fix this by starting an auxiliary event loop for
such cases.

Closes #7434

NO_DOC=bugfix

(cherry picked from commit cdd5674c)