BUG #5297: Add XATMI C API

The following bug has been logged online:

Bug reference:      5297
Logged by:          Juergen Weber
Email address:      weberjn@gmail.com
PostgreSQL version: 8
Operating system:   Linux
Description:        Add XATMI C API
Details:

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)