For rationale, see #6787
Extensions migrated in part 4:
* simplexml
* skeleton
* soap
* spl
* sqlite3
* sysvmsg
* sysvsem
* tidy - also removed a check for an ancient dependency version
shane and rob richards. there were a lot of race conditions with regards
to the way namespaces have been handled in XML documents so far, this new
method removes the race conditions, and gives me a real reason to add the
children() and attributes() methods.