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/notify/notify.c

@@ -24,6 +24,7 @@
 #include "parse-util.h"
 #include "pretty-print.h"
 #include "process-util.h"
+#include "signal-util.h"
 #include "socket-util.h"
 #include "string-util.h"
 #include "strv.h"