Change `poll`'s timeout from `c_int` to `Option<&Timespec>`. #4642

Job	Run time
Check --no-default-features (nightly)	52s
Check nightly-only targets (nightly)	24s
Check (stable)	6m 15s
Check (nightly)	6m 6s
Check selected Tier 3 platforms (nightly)	4m 52s
Rustfmt	8s
Check (1.63)	10m 1s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 14s
Test rustix_use_experimental_asm (s390x-linux)	1m 9s
Test use-libc (ubuntu)	40s
Test (ubuntu)	44s
Test use-libc (i686-linux)	1m 11s
Test (ubuntu-20.04)	43s
Test use-libc (aarch64-linux)	1m 8s
Test (i686-linux)	2m 15s
Test use-libc (powerpc64le-linux)	1m 42s
Test (aarch64-linux)	2m 24s
Test use-libc (riscv64-linux)	1m 49s
Test (powerpc64le-linux)	1m 52s
Test use-libc (s390x-linux)	1m 56s
Test (riscv64-linux)	1m 12s
Test use-libc (arm-linux)	2m 11s
Test (s390x-linux)	1m 12s
Test (arm-linux)	1m 19s
Test (ubuntu-stable)	37s
Test (i686-linux-stable)	1m 13s
Test (aarch64-linux-stable)	1m 8s
Test (riscv64-linux-stable)	1m 5s
Test (s390x-linux-stable)	2m 21s
Test (powerpc64le-linux-stable)	1m 5s
Test (arm-linux-stable)	1m 26s
Test (ubuntu-1.63)	2m 29s
Test (i686-linux-1.63)	3m 10s
Test (aarch64-linux-1.63)	4m 7s
Test (riscv64-linux-1.63)	2m 56s
Test (s390x-linux-1.63)	5m 8s
Test (powerpc64le-linux-1.63)	2m 57s
Test (arm-linux-1.63)	4m 46s
Test (macos-latest)	47s
Test (macos-13)	45s
Test (windows)	55s
Test (windows-2019)	50s
Test (musl)	48s
	1h 31m 52s

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