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/shared/conf-parser.c

@@ -10,7 +10,7 @@
 #include "alloc-util.h"
 #include "conf-files.h"
 #include "conf-parser.h"
-#include "def.h"
+#include "constants.h"
 #include "dns-domain.h"
 #include "escape.h"
 #include "ether-addr-util.h"