Add support for seekdir(). #4486

Job	Run time
Check selected Tier 3 platforms (nightly)	33s
Check --no-default-features (nightly)	58s
Check (stable)	2m 22s
Check nightly-only targets (nightly)	21s
Check (nightly)	2m 24s
Rustfmt	14s
Check (1.63)	2m 38s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 4s
Test rustix_use_experimental_asm (s390x-linux)	1m 6s
Test (ubuntu)	37s
Test (ubuntu-20.04)	38s
Test use-libc (ubuntu)	44s
Test (i686-linux)	26s
Test use-libc (i686-linux)	37s
Test use-libc (aarch64-linux)	52s
Test (aarch64-linux)	28s
Test use-libc (powerpc64le-linux)	53s
Test (powerpc64le-linux)	32s
Test use-libc (riscv64-linux)	52s
Test (riscv64-linux)	26s
Test use-libc (s390x-linux)	52s
Test (s390x-linux)	37s
Test use-libc (arm-linux)	39s
Test (arm-linux)	29s
Test (ubuntu-stable)	37s
Test (i686-linux-stable)	36s
Test (aarch64-linux-stable)	40s
Test (riscv64-linux-stable)	38s
Test (s390x-linux-stable)	26s
Test (powerpc64le-linux-stable)	32s
Test (arm-linux-stable)	30s
Test (ubuntu-1.63)	37s
Test (i686-linux-1.63)	29s
Test (aarch64-linux-1.63)	31s
Test (riscv64-linux-1.63)	33s
Test (s390x-linux-1.63)	29s
Test (powerpc64le-linux-1.63)	30s
Test (arm-linux-1.63)	38s
Test (macos-latest)	23s
Test (macos-13)	39s
Test (windows)	22s
Test (windows-2019)	23s
Test (musl)	38s
	31m 33s

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