[$] New system calls: pidfd_open() and close_range()

The linux-kernel mailing list has recently seen more than the usual amount
of traffic proposing new system calls. LWN is endeavoring to catch up with
that stream, starting with a couple of proposals for the management of file
descriptors. pidfd_open() is a new way to create a 'pidfd' file
descriptor that refers to a process in the system, while
close_range() is an efficient way to close many open descriptors
with a single call.