Add temp_file_limit GUC parameter to constrain temporary file space usage.

The limit is enforced against the total amount of temp file space used by
each session.

Mark Kirkwood, reviewed by Cédric Villemain and Tatsuo Ishii

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

@@ -119,6 +119,11 @@
 #maintenance_work_mem = 16MB		# min 1MB
 #max_stack_depth = 2MB			# min 100kB
 
+# - Disk -
+
+#temp_file_limit = -1			# limits per-session temp file space
+					# in kB, or -1 for no limit
+
 # - Kernel Resource Usage -
 
 #max_files_per_process = 1000		# min 25