We only need to keep track of existing PPPoS interfaces if PPPoS
is not the only enabled protocol.
PPP CORE does not have callbacks pointers for all PPPoS callbacks
which should actually be required for PPPoS (VJ config, asyncmap, ...),
there is too much callbacks to create and PPPoS must be kept light,
especially for users who are only using PPPoS.
But there is a drawback, PPP CORE does not know which
lower protocols it is talking to thanks to the abstraction,
therefore if PPPoS is enabled as well as PPPoE or PPPoL2TP there
might be situation where PPP CORE calls pppos_ config functions
on interfaces which are NOT PPPoS one. This is very unlikely to
happen because protocols not supported by PPPoE or PPPoL2TP are
disabled at LCP/IPCP negotiation but we are better safe than sorry.
So we check if passed PPP pointer to PPPoS configuration functions
is a PPPoS interface by checking against a linked list of existing
PPPoS interfaces.
We don't have callbacks which can be set or cleared for PPPoS
configuration, there is too much callbacks to create and PPPoS must be
kept light, therefore PPPoS functions can be called when PPP core
configure a PPPoE or PPPoL2TP interface, this is very unlikely to
happens because protocols not supported by PPPoE or PPPoL2TP are
disabled at LCP/IPCP negotiation but being safe is still better.
Check if passed PPP pointer to PPPoS configuration functions is a PPPoS
interface by using a linked list of exiting PPPoS interfaces.
