manager: perform objective->shutdown_verb mapping locally

manager: perform objective->shutdown_verb mapping locally

This is a small cleanup removing the need for the spurious
*ret_shutdown_verb argument on invoke_main_loop() while moving
the MANAGER_OBJECTIVE::shutdown_verb string mapping local to
where it actually gets added to the shutdown argv in
become_shutdown().

This also eliminates the need for the several clearings of
*ret_shutdown_argv, and the streq() branches of shutdown_verb in
favor of plain equality tests against the objective value.

Nothing functionally has been changed.