Allow the wal_buffers setting to be auto-tuned to a reasonable value.

If wal_buffers is initially set to -1 (which is now the default), it's
replaced by 1/32nd of shared_buffers, with a minimum of 8 (the old default)
and a maximum of the XLOG segment size.  The allowed range for manual
settings is still from 4 up to whatever will fit in shared memory.

Greg Smith, with implementation correction by me.

src/backend/utils/misc/guc.c

@@ -1765,7 +1765,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_XBLOCKS
 		},
 		&XLOGbuffers,
-		8, 4, INT_MAX, NULL, NULL
+		-1, -1, INT_MAX, NULL, NULL
 	},
 
 	{

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

@@ -162,7 +162,7 @@
 					#   fsync_writethrough
 					#   open_sync
 #full_page_writes = on			# recover from partial page writes
-#wal_buffers = 64kB			# min 32kB
+#wal_buffers = -1			# min 32kB, -1 sets based on shared_buffers
 					# (change requires restart)
 #wal_writer_delay = 200ms		# 1-10000 milliseconds