Make the temporary directory for pgstat files configurable by the GUC

variable stats_temp_directory, instead of requiring the admin to
mount/symlink the pg_stat_tmp directory manually.

For now the config variable is PGC_POSTMASTER. Room for further improvment
that would allow it to be changed on-the-fly.

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

@@ -366,6 +366,7 @@
 #track_functions = none			# none, pl, all
 #track_activity_query_size = 1024
 #update_process_title = on
+#stats_temp_directory = 'pg_stat_tmp'
 
 
 # - Statistics Monitoring -