fn:siblings() on parentless nodes

[michaelhkay]

It feels rather odd that the result of fn:siblings() should include the argument node, except when it is parentless.

If fn:siblings were defined as preceding-sibling::node() | self::node() | following-sibling::node(), then the start node would be included even if it is parentless -- and even if it is an attribute or namespace node.

[michaelhkay]

This also affects the definitions of the following-sibling-or-self and preceding-sibling-or-self axes which are defined in term of the siblings() function.