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/test/test-ellipsize.c

@@ -3,7 +3,7 @@
 #include <stdio.h>
 
 #include "alloc-util.h"
-#include "def.h"
+#include "constants.h"
 #include "string-util.h"
 #include "strv.h"
 #include "terminal-util.h"

src/test/test-exec-util.c

@@ -7,7 +7,7 @@
 
 #include "alloc-util.h"
 #include "copy.h"
-#include "def.h"
+#include "constants.h"
 #include "env-util.h"
 #include "exec-util.h"
 #include "fd-util.h"

src/test/test-mountpoint-util.c

@@ -5,7 +5,7 @@
 #include <unistd.h>
 
 #include "alloc-util.h"
-#include "def.h"
+#include "constants.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "hashmap.h"