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/backlight/backlight.c

@@ -7,6 +7,7 @@
 #include "sd-device.h"
 
 #include "alloc-util.h"
+#include "argv-util.h"
 #include "device-util.h"
 #include "escape.h"
 #include "fileio.h"