util-lib: move main() definition macros to its own header file

This way, we can extend the macro a bit with stuff pulled in from other
headers without this affecting everything which pulls in macro.h, which
is one of our most basic headers.

This is just refactoring, no change in behaviour, in prepartion for
later changes.

src/debug-generator/debug-generator.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
 #include "alloc-util.h"
+#include "main-func.h"
 #include "mkdir.h"
 #include "parse-util.h"
 #include "proc-cmdline.h"