SET TRANSACTION conformance to SQL:2003

Short patch to implement SQL standard behaviour of the SET TRANSACTION
command. According to their publically accessible docs, I note that
MySQL and Ingres correctly implement this behaviour, SQLServer does this
also (and more), while Oracle and DB2 do so since they use implicit
transaction blocks.

Docs page updated.

Manual tests OK. There isn't a test suite for SET TRANSACTION, so I
haven't created one just for this. Behaviour is shown below; the SET
command sets the session characteristics of the *next* transaction, when
executed outside of a transaction block.

postgres=# show transaction_read_only;
 transaction_read_only
-----------------------
 off
(1 row)

postgres=# set transaction read only;
SET
postgres=# begin;
BEGIN
postgres=# show transaction_read_only;
 transaction_read_only
-----------------------
 on
(1 row)

postgres=# commit;
COMMIT


--
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com