[$] Two pidfd tweaks: PIDFD_GET_INFO and PIDFD_SELF

The pidfd mechanism, which uses file descriptors to refer to processes in
an unambiguous and race-free way, <a href="https://lwn.net/Articles/773459/" rel="nofollow">was first
introduced</a> in 2018.  Since then, the interface has https://lwn.net/Articles/794707/
, but 
development has slowed over time as the interface has matured.  There are,
however, a couple of patches in circulation that are meant to make working
with pidfds simpler in some situations.

https://lwn.net/Articles/992991/