Unify calling conventions for postgres/postmaster sub-main functions

There was a wild mix of calling conventions: Some were declared to
return void and didn't return, some returned an int exit code, some
claimed to return an exit code, which the callers checked, but
actually never returned, and so on.

Now all of these functions are declared to return void and decorated
with attribute noreturn and don't return.  That's easiest, and most
code already worked that way.