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/systemctl/systemctl.c

@@ -6,6 +6,7 @@
 
 #include "sd-daemon.h"
 
+#include "argv-util.h"
 #include "build.h"
 #include "bus-util.h"
 #include "capsule-util.h"