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/userdb/userwork.c

@@ -6,7 +6,9 @@
 #include "sd-daemon.h"
 #include "sd-varlink.h"
 
+#include "argv-util.h"
 #include "env-util.h"
+#include "errno-util.h"
 #include "fd-util.h"
 #include "group-record.h"
 #include "io-util.h"