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/sysupdate/sysupdate.c

@@ -8,7 +8,7 @@
 #include "bus-locator.h"
 #include "chase-symlinks.h"
 #include "conf-files.h"
-#include "def.h"
+#include "constants.h"
 #include "dirent-util.h"
 #include "dissect-image.h"
 #include "fd-util.h"