sys/socket: accept interface index in `Ipv6MembershipRequest::new` #2117

Job	Run time
macos	30s
macos-aarch64	23s
solaris (x86_64-pc-solaris)	4m 30s
minver	16s
x86_64_linux_native_builds (x86_64-unknown-linux-musl)	18s
x86_64_linux_native_builds (x86_64-unknown-linux-gnu)	13s
rustfmt	9s
aarch64_linux_native_builds (aarch64-unknown-linux-musl)	17s
rust_stable	1m 51s
tier3 (x86_64-unknown-haiku)	41s
tier3 (armv7-unknown-linux-uclibceabihf)	48s
tier3 (x86_64-unknown-openbsd)	44s
tier3 (x86_64-unknown-dragonfly)	41s
aarch64_linux_native_builds (aarch64-unknown-linux-gnu)	15s
cross	-1s
cross_compiling	-1s
redox	-1s
	11m 33s

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