Rename formats-util.h to format-util.h

We don't have plural in the name of any other -util files and this
inconsistency trips me up every time I try to type this file name
from memory. "formats-util" is even hard to pronounce.

src/initctl/initctl.c

@@ -31,7 +31,7 @@
 #include "bus-util.h"
 #include "def.h"
 #include "fd-util.h"
-#include "formats-util.h"
+#include "format-util.h"
 #include "initreq.h"
 #include "list.h"
 #include "log.h"