Add support for seekdir(). #4485

Job	Run time
Check --no-default-features (nightly)	57s
Check selected Tier 3 platforms (nightly)	1m 16s
Check (stable)	2m 5s
Check nightly-only targets (nightly)	22s
Check (nightly)	2m 12s
Rustfmt	12s
Check (1.63)	2m 9s
Test (ubuntu)	41s
Test use-libc (ubuntu)	45s
Test (ubuntu-20.04)	41s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 8s
Test rustix_use_experimental_asm (s390x-linux)	1m 7s
Test (i686-linux)	35s
Test use-libc (i686-linux)	39s
Test use-libc (aarch64-linux)	54s
Test use-libc (powerpc64le-linux)	53s
Test use-libc (riscv64-linux)	53s
Test (aarch64-linux)	45s
Test use-libc (s390x-linux)	53s
Test (powerpc64le-linux)	31s
Test use-libc (arm-linux)	38s
Test (riscv64-linux)	28s
Test (s390x-linux)	40s
Test (arm-linux)	43s
Test (ubuntu-stable)	38s
Test (i686-linux-stable)	40s
Test (aarch64-linux-stable)	43s
Test (riscv64-linux-stable)	41s
Test (s390x-linux-stable)	41s
Test (powerpc64le-linux-stable)	41s
Test (arm-linux-stable)	33s
Test (ubuntu-1.63)	37s
Test (i686-linux-1.63)	42s
Test (aarch64-linux-1.63)	42s
Test (riscv64-linux-1.63)	40s
Test (s390x-linux-1.63)	40s
Test (powerpc64le-linux-1.63)	32s
Test (arm-linux-1.63)	43s
Test (macos-latest)	25s
Test (macos-13)	41s
Test (windows)	27s
Test (windows-2019)	28s
Test (musl)	43s
	34m 4s

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