Logging transaction IDs for DDL.

I just noticed a problem with log_statement = 'ddl' and log_line_prefix
containing '%x'.  If the statement is the first in the transaction, it
will be logged before it is executed, and more importantly, before a
transaction ID is assigned.  That means that %x will be 0.

If the administrator has configured postgres like this in order to ease
PITR on bad DDL, it's actually of no use whatsoever and they'll have to
use pg_waldump to try to figure things out.

PFA a simple patch that I hope addresses the issue in a clean way.  It
also handles the same problem for 'mod'.

I'm undecided whether this is a bugfix or an improvement.  I'm leaning
towards bugfix.
-- 
Vik Fearing                                          +33 6 46 75 15 36
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support