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/kernel-install/kernel-install.c

@@ -4,6 +4,7 @@
 #include <stdbool.h>
 #include <sys/utsname.h>
 
+#include "argv-util.h"
 #include "boot-entry.h"
 #include "build.h"
 #include "chase.h"