kern/155736: [pf] [altq] borrow from parent queue does not work with cbq

[Marcin Wisnicki]

HFSC can be used as a workaround since it can express everything that
is possible with CBQ:

altq on $lan_if hfsc bandwidth 100Mb queue { qLocal, qExt }
queue qLocal bandwidth 90Mb hfsc(default)
queue qExt bandwidth 10Mb hfsc(upperlimit 10Mb) { qExtFast, qExtSlow }
queue  qExtFast priority 7 bandwidth 80%
queue  qExtSlow priority 1 bandwidth 20% hfsc(red)