Add a new `ReturnFlags` type for flags returned from `recvmsg` #4649

Job	Run time
Check nightly-only targets (nightly)	22s
Check (stable)	6m 9s
Check (nightly)	6m 3s
Check --no-default-features (nightly)	57s
Check selected Tier 3 platforms (nightly)	3m 2s
Check (1.63)	10m 14s
Rustfmt	7s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 14s
Test use-libc (ubuntu)	42s
Test rustix_use_experimental_asm (s390x-linux)	1m 20s
Test (ubuntu)	35s
Test use-libc (i686-linux)	1m 3s
Test use-libc (aarch64-linux)	1m 5s
Test (ubuntu-20.04)	35s
Test (i686-linux)	36s
Test (aarch64-linux)	32s
Test use-libc (powerpc64le-linux)	1m 14s
Test (powerpc64le-linux)	34s
Test use-libc (riscv64-linux)	1m 5s
Test (riscv64-linux)	36s
Test use-libc (s390x-linux)	1m 19s
Test (s390x-linux)	35s
Test use-libc (arm-linux)	2m 17s
Test (arm-linux)	34s
Test (ubuntu-stable)	35s
Test (i686-linux-stable)	36s
Test (aarch64-linux-stable)	36s
Test (riscv64-linux-stable)	34s
Test (s390x-linux-stable)	28s
Test (powerpc64le-linux-stable)	29s
Test (arm-linux-stable)	30s
Test (ubuntu-1.63)	35s
Test (i686-linux-1.63)	37s
Test (aarch64-linux-1.63)	37s
Test (riscv64-linux-1.63)	32s
Test (s390x-linux-1.63)	34s
Test (powerpc64le-linux-1.63)	35s
Test (arm-linux-1.63)	36s
Test (macos-latest)	21s
Test (macos-13)	37s
Test (windows)	21s
Test (windows-2019)	22s
Test (musl)	38s
	53m 3s

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