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/libsystemd/sd-bus/bus-internal.h

@@ -8,7 +8,7 @@
 #include "bus-error.h"
 #include "bus-kernel.h"
 #include "bus-match.h"
-#include "def.h"
+#include "constants.h"
 #include "hashmap.h"
 #include "list.h"
 #include "prioq.h"

src/libsystemd/sd-bus/sd-bus.c

@@ -27,7 +27,7 @@
 #include "bus-track.h"
 #include "bus-type.h"
 #include "cgroup-util.h"
-#include "def.h"
+#include "constants.h"
 #include "errno-util.h"
 #include "fd-util.h"
 #include "glyph-util.h"

src/libsystemd/sd-bus/test-bus-benchmark.c

@@ -8,7 +8,7 @@
 #include "alloc-util.h"
 #include "bus-internal.h"
 #include "bus-kernel.h"
-#include "def.h"
+#include "constants.h"
 #include "fd-util.h"
 #include "missing_resource.h"
 #include "string-util.h"

src/libsystemd/sd-hwdb/hwdb-internal.h

@@ -4,7 +4,7 @@
 #include <stdint.h>
 #include <sys/stat.h>
 
-#include "def.h"
+#include "constants.h"
 #include "hashmap.h"
 #include "sparse-endian.h"