process-util: be more careful with pidfd_get_pid() special cases

process-util: be more careful with pidfd_get_pid() special cases

Let's be more careful with generating error codes for (expected) error
causes.

This does not introduce new error conditions, it just changes what we
return under specific cases, to make things nicely recognizable in each
case. Most importantly this detects if fdinfo reports a pid of "-1" for
pidfds with processes that are already reaped (and thus have no PID
anymore)

None of our current users care about these error codes, but let's get
this right for the future.