pgsql: Code review for txid patch: add binary I/O functions, avoid

Log Message:
-----------
Code review for txid patch: add binary I/O functions, avoid dependence
on SerializableSnapshot, minor other cleanup.  Marko Kreen, some further
editorialization by me.

Modified Files:
--------------
    pgsql/contrib/txid:
        Makefile (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/txid/Makefile?r1=1.1&r2=1.2)
        README.txid (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/txid/README.txid?r1=1.1&r2=1.2)
        txid.c (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/txid/txid.c?r1=1.3&r2=1.4)
        txid.sql.in (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/txid/txid.sql.in?r1=1.1&r2=1.2)
        uninstall_txid.sql (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/txid/uninstall_txid.sql?r1=1.1&r2=1.2)