Simple ping6 application for testing purpose.

The user should wait for the prompt and input the ping6 command
followed by the destination IPv6 address. The address should not use
any abbreviated form (the application does very little input format 
check). 

E.g. of a correct command:
> ping6 fe80:0000:0000:0000:02bd:07ff:fee2:1c00

The ping6 application will then send PING6_NB = 5 ping request packets.
