linux: Add pidfd_getfd

This was added on Linux 5.6 (8649c322f75c96e7ced2fec201e123b2b073bf09)
as a way to retrieve a file descriptors for another process though
pidfd (created either with CLONE_PIDFD or pidfd_getfd).  The
functionality is similar to recvmmsg SCM_RIGHTS.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>