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/fstab-generator/fstab-generator.c

@@ -5,9 +5,11 @@
 #include <unistd.h>
 
 #include "alloc-util.h"
+#include "argv-util.h"
 #include "bus-error.h"
 #include "bus-locator.h"
 #include "bus-unit-util.h"
+#include "bus-util.h"
 #include "chase.h"
 #include "creds-util.h"
 #include "efi-loader.h"