Support having multiple Unix-domain sockets per postmaster.

Replace unix_socket_directory with unix_socket_directories, which is a list
of socket directories, and adjust postmaster's code to allow zero or more
Unix-domain sockets to be created.

This is mostly a straightforward change, but since the Unix sockets ought
to be created after the TCP/IP sockets for safety reasons (better chance
of detecting a port number conflict), AddToDataDirLockFile needs to be
fixed to support out-of-order updates of data directory lockfile lines.
That's a change that had been foreseen to be necessary someday anyway.

Honza Horak, reviewed and revised by Tom Lane

src/backend/utils/adt/varlena.c

@@ -2446,6 +2446,116 @@ SplitIdentifierString(char *rawstring, char separator,
 }
 
 
+/*
+ * SplitDirectoriesString --- parse a string containing directory names
+ *
+ * This is similar to SplitIdentifierString, except that the parsing
+ * rules are meant to handle pathnames instead of identifiers: there is
+ * no downcasing, the max length is MAXPGPATH-1, and we apply
+ * canonicalize_path() to each extracted string.  Because of the last,
+ * the returned strings are separately palloc'd rather than being
+ * pointers into rawstring --- but we still scribble on rawstring.
+ *
+ * Inputs:
+ *	rawstring: the input string; must be modifiable!
+ *	separator: the separator punctuation expected between directories
+ *			   (typically ',' or ';').	Whitespace may also appear around
+ *			   directories.
+ * Outputs:
+ *	namelist: filled with a palloc'd list of directory names.
+ *			  Caller should list_free_deep() this even on error return.
+ *
+ * Returns TRUE if okay, FALSE if there is a syntax error in the string.
+ *
+ * Note that an empty string is considered okay here.
+ */
+bool
+SplitDirectoriesString(char *rawstring, char separator,
+					   List **namelist)
+{
+	char	   *nextp = rawstring;
+	bool		done = false;
+
+	*namelist = NIL;
+
+	while (isspace((unsigned char) *nextp))
+		nextp++;				/* skip leading whitespace */
+
+	if (*nextp == '\0')
+		return true;			/* allow empty string */
+
+	/* At the top of the loop, we are at start of a new directory. */
+	do
+	{
+		char	   *curname;
+		char	   *endp;
+
+		if (*nextp == '\"')
+		{
+			/* Quoted name --- collapse quote-quote pairs */
+			curname = nextp + 1;
+			for (;;)
+			{
+				endp = strchr(nextp + 1, '\"');
+				if (endp == NULL)
+					return false;		/* mismatched quotes */
+				if (endp[1] != '\"')
+					break;		/* found end of quoted name */
+				/* Collapse adjacent quotes into one quote, and look again */
+				memmove(endp, endp + 1, strlen(endp));
+				nextp = endp;
+			}
+			/* endp now points at the terminating quote */
+			nextp = endp + 1;
+		}
+		else
+		{
+			/* Unquoted name --- extends to separator or whitespace */
+			curname = nextp;
+			while (*nextp && *nextp != separator &&
+				   !isspace((unsigned char) *nextp))
+				nextp++;
+			endp = nextp;
+			if (curname == nextp)
+				return false;	/* empty unquoted name not allowed */
+		}
+
+		while (isspace((unsigned char) *nextp))
+			nextp++;			/* skip trailing whitespace */
+
+		if (*nextp == separator)
+		{
+			nextp++;
+			while (isspace((unsigned char) *nextp))
+				nextp++;		/* skip leading whitespace for next */
+			/* we expect another name, so done remains false */
+		}
+		else if (*nextp == '\0')
+			done = true;
+		else
+			return false;		/* invalid syntax */
+
+		/* Now safe to overwrite separator with a null */
+		*endp = '\0';
+
+		/* Truncate path if it's overlength */
+		if (strlen(curname) >= MAXPGPATH)
+			curname[MAXPGPATH - 1] = '\0';
+
+		/*
+		 * Finished isolating current name --- add it to list
+		 */
+		curname = pstrdup(curname);
+		canonicalize_path(curname);
+		*namelist = lappend(*namelist, curname);
+
+		/* Loop back if we didn't reach end of string */
+	} while (!done);
+
+	return true;
+}
+
+
 /*****************************************************************************
  *	Comparison Functions used for bytea
  *