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/core/mount.c

@@ -28,7 +28,7 @@
 #include "dbus-mount.h"
 #include "escape.h"
 #include "exit-status.h"
-#include "formats-util.h"
+#include "format-util.h"
 #include "fstab-util.h"
 #include "log.h"
 #include "manager.h"