YPORT: retransmit identical data on uip_rexmit()

up until now, yport does not retransmit identical data on uip_rexmit()==TRUE. So when further serial data is received and the ACK-packet is lost or comes late, but the packet was correctly received, data gets lost.

+ service the every heartbeat to cope with variable data flow

pool serial data to avoid many small ethernet packets
+ send if buffer if filled up to one quater
+ send buffer if linefeed is received (does no harm on binary data)
+ send if one second has passed and we have serial data

+ fixed some typos