Pull up following revision(s) (requested by knakahara in ticket #730):

sys/net/if_l2tp.c: revision 1.22
sys/net/if_l2tp.c: revision 1.23

Improve comment. Pointed out by maxv@n.o, thanks.

Fix previous my mistake and odd unaligned case. Pointed out by maxv@n.o, thanks.
It must be rare case to be required this copy routine...

(martin)