pgsql: XPath fixes: - Function renamed to "xpath".

Log Message:
-----------
XPath fixes:
 - Function renamed to "xpath".
 - Function is now strict, per discussion.
 - Return empty array in case when XPath expression detects nothing
   (previously, NULL was returned in such case), per discussion.
 - (bugfix) Work with fragments with prologue: select xpath('/a',
   '<?xml version="1.0"?><a /><b />'); // now XML datum is always wrapped
   with dummy <x>...</x>, XML prologue simply goes away (if any).
 - Some cleanup.

Nikolay Samokhvalov

Some code cleanup and documentation work by myself.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        datatype.sgml (r1.200 -> r1.201)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.200&r2=1.201)
        func.sgml (r1.379 -> r1.380)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.379&r2=1.380)
    pgsql/src/backend/utils/adt:
        xml.c (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.42&r2=1.43)
    pgsql/src/include/catalog:
        catversion.h (r1.406 -> r1.407)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.406&r2=1.407)
        pg_proc.h (r1.455 -> r1.456)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.455&r2=1.456)
    pgsql/src/include/utils:
        xml.h (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h.diff?r1=1.18&r2=1.19)
    pgsql/src/test/regress/expected:
        xml.out (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml.out.diff?r1=1.17&r2=1.18)
        xml_1.out (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml_1.out.diff?r1=1.16&r2=1.17)
    pgsql/src/test/regress/sql:
        xml.sql (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/xml.sql.diff?r1=1.14&r2=1.15)