pgsql: Add an xpath_exists() function.

Log Message:
-----------
Add an xpath_exists() function.  This is equivalent to XMLEXISTS except that
it offers support for namespace mapping.

Mike Fowler, reviewed by David Fetter

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.524 -> r1.525)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.524&r2=1.525)
    pgsql/src/backend/utils/adt:
        xml.c (r1.99 -> r1.100)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.99&r2=1.100)
    pgsql/src/include/catalog:
        catversion.h (r1.592 -> r1.593)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.592&r2=1.593)
        pg_proc.h (r1.576 -> r1.577)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.576&r2=1.577)
    pgsql/src/include/utils:
        xml.h (r1.32 -> r1.33)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h?r1=1.32&r2=1.33)
    pgsql/src/test/regress/expected:
        xml.out (r1.26 -> r1.27)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml.out?r1=1.26&r2=1.27)
        xml_1.out (r1.27 -> r1.28)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml_1.out?r1=1.27&r2=1.28)
    pgsql/src/test/regress/sql:
        xml.sql (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/xml.sql?r1=1.20&r2=1.21)