main-func: Reduce transitive includes

Let's move some logic from _DEFINE_MAIN_FUNCTION() and other places
in main-func.h into functions that we implement in main-func.c to
allow moving some included headers from the header to the .c file.

src/timedate/timedated.c

@@ -20,6 +20,7 @@
 #include "bus-message-util.h"
 #include "bus-polkit.h"
 #include "bus-unit-util.h"
+#include "bus-util.h"
 #include "clock-util.h"
 #include "conf-files.h"
 #include "constants.h"