our own xmlrpc build is about supporting <nil/> - aka allow_none=True

http://gggeek.github.io/phpxmlrpc/ states that the <nil/> extension
is supported by mainstream - and probably has been for a while
- this change is about dropping our own stuff
and using mainstream xmlrpc instead
pretext is fedora25 b/c this won’t build anymore
but again it’s probably OK w/ f24 and way before