[$] Namespace reference counting and listns()

The kernel's namespaces feature is, among
other things, a key part of the implementation of containers. Like much in
the kernel, though, the namespace API evolved over time; there was no
design at the outset. As a result, this API has some rough edges and
missing features. Christian Brauner is working to straighten out the
namespace situation somewhat with this
daunting 72-part patch series that, among other things, adds a new
system call to allow user space to query the namespaces present on the
system.