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/journal-remote/journal-remote-main.c

@@ -7,8 +7,8 @@
 
 #include "build.h"
 #include "conf-parser.h"
+#include "constants.h"
 #include "daemon-util.h"
-#include "def.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "journal-remote-write.h"

src/journal-remote/journal-remote.c

@@ -10,7 +10,7 @@
 
 #include "af-list.h"
 #include "alloc-util.h"
-#include "def.h"
+#include "constants.h"
 #include "errno-util.h"
 #include "escape.h"
 #include "fd-util.h"

src/journal-remote/journal-upload.c

@@ -12,8 +12,8 @@
 #include "alloc-util.h"
 #include "build.h"
 #include "conf-parser.h"
+#include "constants.h"
 #include "daemon-util.h"
-#include "def.h"
 #include "env-file.h"
 #include "fd-util.h"
 #include "fileio.h"