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/nspawn/nspawn.c

@@ -16,6 +16,10 @@
 #include <termios.h>
 #include <unistd.h>
 
+#if HAVE_SELINUX
+#include <selinux/selinux.h>
+#endif
+
 #include "sd-bus.h"
 #include "sd-daemon.h"
 #include "sd-id128.h"