Return PID from waitpid #3780

Job	Run time
Check (stable)	2m 42s
Check (nightly)	2m 45s
Rustfmt	4s
Check (1.63)	2m 59s
Check --no-default-features (nightly)	57s
Check nightly-only targets (nightly)	23s
Check selected Tier 3 platforms (nightly)	2m 45s
Test (ubuntu)	39s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 1s
Test (ubuntu-20.04)	41s
Test (i686-linux)	56s
Test use-libc (ubuntu)	36s
Test (aarch64-linux)	54s
Test use-libc (i686-linux)	41s
Test (powerpc64le-linux)	56s
Test use-libc (aarch64-linux)	59s
Test (riscv64-linux)	55s
Test use-libc (powerpc64le-linux)	56s
Test (s390x-linux)	55s
Test use-libc (riscv64-linux)	57s
Test (arm-linux)	57s
Test use-libc (arm-linux)	43s
Test (ubuntu-stable)	36s
Test (i686-linux-stable)	51s
Test (aarch64-linux-stable)	54s
Test (riscv64-linux-stable)	54s
Test (s390x-linux-stable)	53s
Test (powerpc64le-linux-stable)	55s
Test (arm-linux-stable)	55s
Test (ubuntu-1.63)	54s
Test (i686-linux-1.63)	53s
Test (aarch64-linux-1.63)	56s
Test (riscv64-linux-1.63)	55s
Test (s390x-linux-1.63)	47s
Test (powerpc64le-linux-1.63)	55s
Test (arm-linux-1.63)	57s
Test (macos-latest)	36s
Test (macos-11)	35s
Test (windows)	39s
Test (windows-2019)	41s
Test (musl)	39s
	40m 46s

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