Shorten string packets

[ghost]

Currently, string packets are always fragmented. Since fragmentation hurts us - our devices do not have enough memory to reassemble more than one packet at any given time, fragments from all other packets are simply dropped - we must not fragment packets if at all possible. Since we currently do send a noticable amount of string packets, these packets should be shortened to something that does not force 6lowpan fragmentation.

Before this can sensibly be done, #210 must be addressed.