Changed main_loop shutdown logic once again. The former mechanism with

Changed main_loop shutdown logic once again. The former mechanism with
a single global main_shutdown variable was too inflexible. I have given
all hooks a possibility to either invoke an immediate shutdown or a "soft"
shutdown which is activated if all hooks agree on it (the idea is that
different parts of a single program can have their local opinion on whether
there is more to do or not).