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/mount/mount-tool.c

@@ -5,6 +5,7 @@
 #include "sd-bus.h"
 #include "sd-device.h"
 
+#include "argv-util.h"
 #include "build.h"
 #include "bus-error.h"
 #include "bus-locator.h"