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/modules-load/modules-load.c

@@ -8,6 +8,7 @@
 #include "build.h"
 #include "conf-files.h"
 #include "constants.h"
+#include "errno-util.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "log.h"