bgpd: transfer raw input buffer to new peer

bgpd: transfer raw input buffer to new peer

During initial session establishment, bgpd performs a "connection
transfer" to a new peer struct if the connection was initiated passively
(i.e. by the remote peer). With the addition of buffered input, I forgot
to transfer the raw input buffer to the new peer. This resulted in
infrequent failures during session handshaking whereby half of a packet
would be thrown away in the middle of a read causing us to send a NOTIFY
for an unsynchronized header. Usually the transfer coincided with a
clean input buffer, hence why it only showed up once in a while.