Change default of log_directory to 'log'

The previous default 'pg_log' might have indicated by its "pg_" prefix
that it is an internal system directory.  The new default is more in
line with the typical naming of directories with user-facing log files.
Together with the renaming of pg_clog and pg_xlog, this should clear up
that difference.

Author: Andreas Karlsson <andreas@proxel.se>

src/backend/utils/misc/guc.c

@@ -3320,7 +3320,7 @@ static struct config_string ConfigureNamesString[] =
 			GUC_SUPERUSER_ONLY
 		},
 		&Log_directory,
-		"pg_log",
+		"log",
 		check_canonical_path, NULL, NULL
 	},
 	{

src/backend/utils/misc/postgresql.conf.sample

@@ -344,7 +344,7 @@
 					# (change requires restart)
 
 # These are only used if logging_collector is on:
-#log_directory = 'pg_log'		# directory where log files are written,
+#log_directory = 'log'			# directory where log files are written,
 					# can be absolute or relative to PGDATA
 #log_filename = 'postgresql-%Y-%m-%d_%H%M%S.log'	# log file name pattern,
 					# can include strftime() escapes