Localize sequence packet checking in SocketZero class

Localize sequence packet checking in SocketZero class

Put sequence packet check directly in the
SocketZero::RawReceive() function, which now localizes all
sequence packet handling and checking, as well as avoids
packet order problems when multiple sockets are in use,
such as with pppob...
FIXME - this localized sequence checking may make
all the calls to CheckSequence() obsolete, which
should probably be cleaned up someday, if so.