Move copydir() prototype into its own header file.

Having this in src/include/port.h makes no sense, now that copydir.c lives
in src/backend/strorage rather than src/port.  Along the way, remove an
obsolete comment from contrib/pg_upgrade that makes reference to the old
location.

src/backend/storage/file/copydir.c

@@ -22,6 +22,7 @@
 #include <unistd.h>
 #include <sys/stat.h>
 
+#include "storage/copydir.h"
 #include "storage/fd.h"
 #include "miscadmin.h"