Pad the ps_status display with nulls, not blanks, on Darwin.

A long time ago, this didn't work nicely, but it seems to work on all recent
versions of OS X.  The blank-pad method is less desirable since it results
in lots of extra space in ps' output.  Per Alexey Klyukin.

src/backend/utils/misc/ps_status.c

@@ -5,7 +5,7 @@
  * to contain some useful information. Mechanism differs wildly across
  * platforms.
  *
- * $PostgreSQL: pgsql/src/backend/utils/misc/ps_status.c,v 1.42 2010/07/06 19:18:59 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/misc/ps_status.c,v 1.43 2010/09/04 17:45:56 tgl Exp $
  *
  * Copyright (c) 2000-2010, PostgreSQL Global Development Group
  * various details abducted from various places
@@ -76,7 +76,7 @@ bool		update_process_title = true;
 
 
 /* Different systems want the buffer padded differently */
-#if defined(_AIX) || defined(__linux__) || defined(__svr4__)
+#if defined(_AIX) || defined(__linux__) || defined(__svr4__) || defined(__darwin__)
 #define PS_PADDING '\0'
 #else
 #define PS_PADDING ' '