Make xmlAddChild unlink the child before insertion. Originally, linked
children would most likely cause tree corruption. The first fix
disallowed linked nodes, but there are cases where insertion of such
nodes could succeed.
Don't abort if the node is already a child of parent. In this case,
the node will be moved to the end of the child list.
After the failed experiment with a static XML namespace, introduce
versions of xmlSearchNs that report malloc failures.
Optimize the no-document case by only adding the XML namespace
declaration if it wasn't found in an ancestor.
