RT6: Update some printings for coherency

RT6: Update some printings for coherency

s/srcrt/RT6/
In upper case (like 'IP6'), via a nd_print_protocol_caps() call.

Before: srcrt (len=48, type=0, segleft=48 [|rt6]
After:  RT6 (len=48, type=0, segleft=48 [|rt6]

rt6 choosen because it's in file name (print-rt6.c), function name
(rt6_print) and set in ndo->ndo_protocol ("rt6").

Moreover:
Print the protocol name even if the packet is truncated at the beginning.
Use nd_print_trunc().