[$] Namespace reference counting and listns()

The kernel's namespaces feature is, amongother things, a key part of the implementation of containers. Like much inthe kernel, though, the namespace API evolved over time; there was nodesign at the outset. As a result, this API has some rough edges andmissing features. Christian Brauner is working to straighten out thenamespace situation somewhat with thisdaunting 72-part patch series that, among other things, adds a newsystem call to allow user space to query the namespaces present on thesystem.