Add a new option for ppp.conf: rad_port_id. It allows to
change the way of what ppp submits to the RADIUS server
as NAS-Port-Id. Possible options are: the PID of the process
owning the corresponding interface, tun(4) interface number,
interface index (as it would get returned by if_nametoindex(3)),
or it's possible to keep the default behavior. Check the ppp(8)
manual page for details.

PR: bin/112764
Submitted by: novel (myself)
Reviewed by: flz
Approved by: flz
MFC after: 1 month