fib: allow for multiple instances of FIB

sys/auto_init/auto_init.c

sys/include/net/fib.h

sys/include/net/gnrc/ipv6.h

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

sys/net/gnrc/network_layer/ndp/node/gnrc_ndp_node.c

sys/net/gnrc/routing/rpl/gnrc_rpl_control_messages.c

sys/net/gnrc/routing/rpl/gnrc_rpl_dodag.c

sys/net/network_layer/fib/fib.c

sys/shell/commands/sc_fib.c

tests/unittests/tests-fib/tests-fib.c