pam-util: include PID in PAM data field id

pam-util: include PID in PAM data field id

Let's systematically avoid sharing cached busses between processes (i.e.
from parent and child after fork()), by including the PID in the field
name.

With that we're never tempted to use a bus object the parent created in
the child.

(Note this is about *use*, not about *destruction*. Destruction needs to
be checked by other means.)