Merge branch 'PR/nsenter-pidfd' of https://github.com/karelzak/util-linux-work

* 'PR/nsenter-pidfd' of https://github.com/karelzak/util-linux-work:
  nsenter: Rewrite --user-parent to use pidfd
  include/pidfd-utils: add namespaces ioctls
  nsenter: reuse pidfd for --net-socket
  nsenter: use macros to access the nsfiles array
  nsenter: use pidfd to enter target namespaces
  nsenter: use separate function to enter namespaces
  nsenter: add functions to enable/disable namespaces