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/machine/machinectl.c

@@ -38,6 +38,7 @@
 #include "hostname-util.h"
 #include "import-util.h"
 #include "in-addr-util.h"
+#include "label-util.h"
 #include "locale-util.h"
 #include "log.h"
 #include "logs-show.h"

src/machine/machined.c

@@ -11,6 +11,7 @@
 #include "bus-locator.h"
 #include "bus-log-control-api.h"
 #include "bus-polkit.h"
+#include "bus-util.h"
 #include "cgroup-util.h"
 #include "common-signal.h"
 #include "daemon-util.h"