Support C/R of pidfd descriptors

pidfds are a feature of the Linux kernel that provide a process with a stable handle on another process. They were introduced to solve the problem of PID recycling. A user can send a signal to the process (pidfd_send_signal syscall), wait for the process (poll() on pidfd) using a pidfd. However, with the current implementation of CRIU, it can't checkpoint/restore a process with pidfd file descriptor(s) opened. So, the proposed project's purpose is to add new functionality to successfully checkpoint/restore any process that has opened pidfd file descriptor(s).