Rename def.h to constants.h

The name "def.h" originates from before the rule of "no needless abbreviations"
was established. Let's rename the file to clarify that it contains a collection
of various semi-related constants.

src/sysusers/sysusers.c

@@ -7,9 +7,9 @@
 #include "build.h"
 #include "chase-symlinks.h"
 #include "conf-files.h"
+#include "constants.h"
 #include "copy.h"
 #include "creds-util.h"
-#include "def.h"
 #include "dissect-image.h"
 #include "env-util.h"
 #include "fd-util.h"