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/shared/install-printf.c

@@ -22,7 +22,7 @@
 #include <string.h>
 #include <unistd.h>
 
-#include "formats-util.h"
+#include "format-util.h"
 #include "install-printf.h"
 #include "install.h"
 #include "macro.h"