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/basic/meson.build

@@ -33,7 +33,7 @@ basic_sources = files(
         'chattr-util.h',
         'conf-files.c',
         'conf-files.h',
-        'def.h',
+        'constants.h',
         'devnum-util.c',
         'devnum-util.h',
         'dirent-util.c',