Make sixlowpan module more coding conventions compliant

sys/net/rpl/etx_beaconing.c

sys/net/rpl/etx_beaconing.h

sys/net/rpl/of0.c

sys/net/rpl/of_mrhof.c

sys/net/rpl/rpl.c

sys/net/rpl/rpl.h

sys/net/rpl/rpl_dodag.c

sys/net/rpl/rpl_dodag.h

sys/net/rpl/rpl_structs.h

sys/net/rpl/trickle.c

sys/net/rpl/trickle.h

sys/net/sixlowpan/border.c

sys/net/sixlowpan/border.h

sys/net/sixlowpan/bordermultiplex.c

sys/net/sixlowpan/bordermultiplex.h

sys/net/sixlowpan/flowcontrol.c

sys/net/sixlowpan/flowcontrol.h

sys/net/sixlowpan/icmp.c

sys/net/sixlowpan/icmp.h

sys/net/sixlowpan/ip.c