[ip6] add API functions to enable multicast looping of a message #20

Job	Run time
expects	32m 10s
packet-verification-1-1-on-1-3	18m 8s
packet-verification-low-power	8m 17s
thread-1-3-gcc-m32	14m 54s
thread-1-3-gcc-m64	15m 31s
thread-1-3-posix	12m 14s
thread-1-3-clang-10-m32	18m 21s
thread-1-3-clang-10-m64	13m 39s
upload-coverage	40s
delete-coverage-artifacts	13s
	2h 14m 7s

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