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/sysusers/sysusers.c

@@ -30,7 +30,7 @@
 #include "def.h"
 #include "fd-util.h"
 #include "fileio-label.h"
-#include "formats-util.h"
+#include "format-util.h"
 #include "hashmap.h"
 #include "path-util.h"
 #include "selinux-util.h"