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/oom/oomctl.c

@@ -9,6 +9,7 @@
 #include "bus-message-util.h"
 #include "log.h"
 #include "main-func.h"
+#include "pager.h"
 #include "pretty-print.h"
 #include "terminal-util.h"
 #include "verbs.h"