Revert exporting of internal GUC variable "data_directory".

This undoes a poorly-thought-out choice in commit 970a18687f, namely
to export guc.c's internal variable data_directory.  The authoritative
variable so far as C code is concerned is DataDir; there is no reason for
anything except specific bits of GUC code to look at the GUC variable.

After yesterday's commits fixing the fsync-on-restart patch, the only
remaining misuse of data_directory was in AlterSystemSetConfigFile(),
which would be much better off just using a relative path anyhow: it's
less code and it doesn't break if the DBA moves the data directory of a
running system, which is a case we've taken some pains over in the past.

This is mostly cosmetic, so no need for a back-patch (and I'd be hesitant
to remove a global variable in stable branches anyway).

src/backend/utils/misc/guc.c

@@ -435,7 +435,6 @@ int			temp_file_limit = -1;
 int			num_temp_buffers = 1024;
 
 char	   *cluster_name = "";
-char	   *data_directory;
 char	   *ConfigFileName;
 char	   *HbaFileName;
 char	   *IdentFileName;
@@ -476,6 +475,7 @@ static char *timezone_string;
 static char *log_timezone_string;
 static char *timezone_abbreviations_string;
 static char *XactIsoLevel_string;
+static char *data_directory;
 static char *session_authorization_string;
 static int	max_function_args;
 static int	max_index_keys;
@@ -6895,11 +6895,11 @@ AlterSystemSetConfigFile(AlterSystemStmt *altersysstmt)
 	}
 
 	/*
-	 * Use data directory as reference path for PG_AUTOCONF_FILENAME and its
-	 * corresponding temporary file.
+	 * PG_AUTOCONF_FILENAME and its corresponding temporary file are always in
+	 * the data directory, so we can reference them by simple relative paths.
 	 */
-	join_path_components(AutoConfFileName, data_directory, PG_AUTOCONF_FILENAME);
-	canonicalize_path(AutoConfFileName);
+	snprintf(AutoConfFileName, sizeof(AutoConfFileName), "%s",
+			 PG_AUTOCONF_FILENAME);
 	snprintf(AutoConfTmpFileName, sizeof(AutoConfTmpFileName), "%s.%s",
 			 AutoConfFileName,
 			 "tmp");

src/include/utils/guc.h

@@ -247,7 +247,6 @@ extern int	temp_file_limit;
 extern int	num_temp_buffers;
 
 extern char *cluster_name;
-extern char *data_directory;
 extern char *ConfigFileName;
 extern char *HbaFileName;
 extern char *IdentFileName;