Stop talking about the "XDG" version of basename()

XDG refers to X Desktop Group, a former name for freedesktop.org.
This group is responsible for specifications like basedirs,
.desktop files and icon naming, but as far as I know, it has never
tried to redefine basename().

I think these references were meant to say XPG (X/Open Portability
Guide), a precursor of POSIX. POSIX is better-known and less easily
confused with XDG, and is how the basename(3) man page describes
the libgen.h version of basename().

The other version of basename() is glibc-specific and is described
in basename(3) as "the GNU version"; specifically mention that
version, to disambiguate.

CODING_STYLE

@@ -210,11 +210,12 @@
   c) recvmsg() must get MSG_CMSG_CLOEXEC set
   d) F_DUPFD_CLOEXEC should be used instead of F_DUPFD, and so on
 
-- We never use the XDG version of basename(). glibc defines it in
-  libgen.h. The only reason to include that file is because dirname()
+- We never use the POSIX version of basename() (which glibc defines it in
+  libgen.h), only the GNU version (which glibc defines in string.h).
+  The only reason to include libgen.h is because dirname()
   is needed. Everytime you need that please immediately undefine
   basename(), and add a comment about it, so that no code ever ends up
-  using the XDG version!
+  using the POSIX version!
 
 - Use the bool type for booleans, not integers. One exception: in public
   headers (i.e those in src/systemd/sd-*.h) use integers after all, as "bool"

src/basic/util.c

@@ -60,8 +60,8 @@
 #include <linux/fs.h>
 
 /* When we include libgen.h because we need dirname() we immediately
- * undefine basename() since libgen.h defines it as a macro to the XDG
- * version which is really broken. */
+ * undefine basename() since libgen.h defines it as a macro to the POSIX
+ * version which is really broken. We prefer GNU basename(). */
 #include <libgen.h>
 #undef basename
 

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

@@ -29,8 +29,8 @@
 #include <sys/prctl.h>
 
 /* When we include libgen.h because we need dirname() we immediately
- * undefine basename() since libgen.h defines it as a macro to the XDG
- * version which is really broken. */
+ * undefine basename() since libgen.h defines it as a macro to the POSIX
+ * version which is really broken. We prefer GNU basename(). */
 #include <libgen.h>
 #undef basename
 

src/machine/machine-dbus.c

@@ -24,8 +24,8 @@
 #include <sys/mount.h>
 
 /* When we include libgen.h because we need dirname() we immediately
- * undefine basename() since libgen.h defines it as a macro to the XDG
- * version which is really broken. */
+ * undefine basename() since libgen.h defines it as a macro to the POSIX
+ * version which is really broken. We prefer GNU basename(). */
 #include <libgen.h>
 #undef basename