Make xpath() do something useful with XPath expressions that return scalars.

Previously, xpath() simply returned an empty array if the expression did
not yield a node set.  This is useless for expressions that return scalars,
such as one with name() at the top level.  Arrange to return the scalar
value as a single-element xml array, instead.  (String values will be
suitably escaped.)

This change will also cause xpath_exists() to return true, not false,
for such expressions.

Florian Pflug, reviewed by Radoslaw Smogura

doc/src/sgml/func.sgml

@@ -9275,6 +9275,8 @@ SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuf
     against the XML value
     <replaceable>xml</replaceable>.  It returns an array of XML values
     corresponding to the node set produced by the XPath expression.
+    If the XPath expression returns a scalar value rather than a node set,
+    a single-element array is returned.
    </para>
 
   <para>