6lowpan: revise the initial commit

The new structures needed to be memset zero in gram.y.  I also
moved the two new members for 6lowpan up a bit in the interface
strucutre to group and comment them.
6lowpan: minor style changes
sllao: clean up readability of sllao in send.c

This code was not commented and it was unclear.  This made it
challenging for 6lowpan code to be added (as the hardware address
is not the same size as standard ethernet).  Hopefully this change
will make the intent of the sllao code clear.