tree-wide: whenever we include libgen.h, immediately undefine basename()

Also, document in adjacent comments and in CODING_STYLE why we do that.

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

@@ -25,10 +25,15 @@
 
 #include <fcntl.h>
 #include <malloc.h>
-#include <libgen.h>
 #include <sys/mman.h>
 #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. */
+#include <libgen.h>
+#undef basename
+
 #include "util.h"
 #include "strv.h"
 #include "memfd-util.h"