Remove the custom interface (and backends and command-line tools)

It has never been used, it's known not to work well for actual networks
(that's why netloc is coming), and it makes the core more complex without
any good reason.

** This commit breaks the ABI **
NEWS update about 2.0, netloc and custom removal
Update some assumptions now that the custom interface is gone

root object always has a cpuset
parent always has a cpuset/nodeset if child has one