Add XATMI C API

Hi,

an XATMI C API should be added to the PostgresQL C client, so one could run
XA transactions under control of a TX monitor. An interesting usecase would
be to use PostgresQL together with the LGPL open source JBoss Blacktie
transaction manager (http://www.jboss.org/blacktie.html)

c.f.
http://archives.postgresql.org/pgsql-bugs/2010-01/msg00236.php

Thanks,
Juergen