pidref: record pidfd inode number in PidRef struct

pidref: record pidfd inode number in PidRef struct

Besides internal comparisons, the inode number of pidfds
might be interesting directly to users, too. In the future
this field should also be exposed, so that it can serve as
a unique identifier of a process (but only for display,
as there's no method to map this back to a pid or pidfd).