pgsql: Rename xml_valid() to xml_is_well_formed(), but provide a

Log Message:
-----------
Rename xml_valid() to xml_is_well_formed(), but provide a temporary
alias with the old name for backwards compatibility.  Per discussion,
the old name is actively wrong because validity and well-formedness
have different meanings in XML.

Modified Files:
--------------
    pgsql/contrib/xml2:
        README.xml2 (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/README.xml2.diff?r1=1.6&r2=1.7)
        pgxml.sql.in (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/pgxml.sql.in.diff?r1=1.8&r2=1.9)
        uninstall_pgxml.sql (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/uninstall_pgxml.sql.diff?r1=1.1&r2=1.2)
        xpath.c (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xpath.c.diff?r1=1.12&r2=1.13)
    pgsql/doc/src/sgml:
        datatype.sgml (r1.174 -> r1.175)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.174&r2=1.175)