Add setter and getter for `multicast_ifv4` and `multicast_ifv6` #4679

Job	Run time
Check nightly-only targets (nightly)	25s
Rustfmt	4s
Check (stable)	6m 13s
Check --no-default-features (nightly)	54s
Check (nightly)	6m 1s
Check selected Tier 3 platforms (nightly)	4m 56s
Check (1.63)	10m 9s
Test (ubuntu)	43s
Test (ubuntu-20.04)	42s
Test (i686-linux)	1m 7s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 3s
Test use-libc (ubuntu)	36s
Test (aarch64-linux)	58s
Test rustix_use_experimental_asm (s390x-linux)	1m 18s
Test use-libc (i686-linux)	1m 3s
Test (powerpc64le-linux)	1m 5s
Test use-libc (aarch64-linux)	52s
Test (riscv64-linux)	1m 5s
Test use-libc (powerpc64le-linux)	1m 6s
Test (s390x-linux)	1m 5s
Test use-libc (riscv64-linux)	1m 6s
Test (arm-linux)	59s
Test use-libc (s390x-linux)	1m 4s
Test (ubuntu-stable)	39s
Test use-libc (arm-linux)	1m 4s
Test (i686-linux-stable)	1m 6s
Test (aarch64-linux-stable)	54s
Test (riscv64-linux-stable)	58s
Test (s390x-linux-stable)	59s
Test (powerpc64le-linux-stable)	55s
Test (arm-linux-stable)	51s
Test (ubuntu-1.63)	1m 5s
Test (i686-linux-1.63)	1m 6s
Test (aarch64-linux-1.63)	1m 7s
Test (riscv64-linux-1.63)	1m 5s
Test (s390x-linux-1.63)	1m 5s
Test (powerpc64le-linux-1.63)	1m 6s
Test (arm-linux-1.63)	1m 6s
Test (macos-latest)	27s
Test (macos-13)	45s
Test (windows)	51s
Test (windows-2019)	50s
Test (musl)	44s
	1h 3m 17s

Provide feedback