Move pg_dump memory routines into pg_dumpmem.c/h and restore common.c

with its original functions.  The previous function migration would
cause too many difficulties in back-patching.

src/bin/pg_dump/pg_backup_null.c

@@ -23,7 +23,7 @@
  */
 
 #include "pg_backup_archiver.h"
-#include "common.h"
+#include "dumpmem.h"
 #include "dumputils.h"
 
 #include <unistd.h>				/* for dup */