Allow single table VACUUM in transaction block

It is a common user annoyance to have a script fail because someone
added a VACUUM, especially when using --single-transaction option.
Fix, so that this works without issue:

BEGIN;
....
VACUUM (ANALYZE) vactst;
....
COMMIT;

Allows both ANALYZE and vacuum of toast tables, but not VACUUM FULL.

When in a xact block, we do not set PROC_IN_VACUUM,
nor update datfrozenxid.

Tests, docs.

--
Simon Riggs                http://www.EnterpriseDB.com/