Implement join_mcast_group_intf funciton #86

Job	Run time
lint	35s
test (libcoap-sys, openssl, msrv)	2m 7s
docs	2m 12s
test (libcoap-sys, openssl, stable)	2m 22s
test (libcoap-sys, openssl, nightly)	2m 17s
test (libcoap-sys, gnutls, msrv)	1m 23s
test (libcoap-sys, gnutls, stable)	46s
test (libcoap-sys, gnutls, nightly)	58s
test (libcoap-sys, tinydtls, msrv)	1m 6s
test (libcoap-sys, tinydtls, stable)	1m 32s
test (libcoap-sys, tinydtls, nightly)	1m 6s
test (libcoap-sys, mbedtls, msrv)	1m 16s
test (libcoap-sys, mbedtls, stable)	1m 6s
test (libcoap-sys, mbedtls, nightly)	1m 16s
test (libcoap-rs, openssl, msrv)	2m 16s
test (libcoap-rs, openssl, stable)	2m 9s
test (libcoap-rs, openssl, nightly)	2m 26s
test (libcoap-rs, gnutls, msrv)	1m 7s
test (libcoap-rs, gnutls, stable)	58s
test (libcoap-rs, gnutls, nightly)	1m 9s
test (libcoap-rs, tinydtls, msrv)	2m 7s
test (libcoap-rs, tinydtls, stable)	54s
test (libcoap-rs, tinydtls, nightly)	1m 21s
test (libcoap-rs, mbedtls, msrv)	1m 8s
test (libcoap-rs, mbedtls, stable)	1m 17s
test (libcoap-rs, mbedtls, nightly)	1m 22s
	38m 16s

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