Add support for seekdir(). #4484

Job	Run time
Check selected Tier 3 platforms (nightly)	1m 16s
Check (stable)	1m 26s
Check nightly-only targets (nightly)	24s
Check --no-default-features (nightly)	25s
Check (nightly)	1m 23s
Check (1.63)	1m 29s
Rustfmt	11s
Test rustix_use_experimental_asm (powerpc64le-linux)	42s
Test rustix_use_experimental_asm (s390x-linux)	37s
Test use-libc (ubuntu)	44s
Test use-libc (i686-linux)	38s
Test (ubuntu)	22s
Test use-libc (aarch64-linux)	49s
Test (ubuntu-20.04)	22s
Test use-libc (powerpc64le-linux)	49s
Test (i686-linux)	27s
Test use-libc (riscv64-linux)	49s
Test (aarch64-linux)	24s
Test use-libc (s390x-linux)	49s
Test (powerpc64le-linux)	25s
Test use-libc (arm-linux)	33s
Test (riscv64-linux)	30s
Test (s390x-linux)	32s
Test (arm-linux)	27s
Test (ubuntu-stable)	23s
Test (i686-linux-stable)	29s
Test (aarch64-linux-stable)	36s
Test (riscv64-linux-stable)	29s
Test (s390x-linux-stable)	34s
Test (powerpc64le-linux-stable)	32s
Test (arm-linux-stable)	23s
Test (ubuntu-1.63)	32s
Test (i686-linux-1.63)	29s
Test (aarch64-linux-1.63)	33s
Test (riscv64-linux-1.63)	27s
Test (s390x-linux-1.63)	25s
Test (powerpc64le-linux-1.63)	25s
Test (arm-linux-1.63)	25s
Test (macos-latest)	32s
Test (macos-13)	34s
Test (windows)	19s
Test (windows-2019)	19s
Test (musl)	18s
	25m 17s

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