Return PID from waitpid #3772

Job	Run time
Check nightly-only targets (nightly)	27s
Check selected Tier 3 platforms (nightly)	2m 49s
Check --no-default-features (nightly)	54s
Check (stable)	3m 11s
Check (nightly)	3m 19s
Rustfmt	14s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 9s
Check (1.63)	3m 29s
Test (ubuntu)	47s
Test (ubuntu-20.04)	44s
Test use-libc (ubuntu)	41s
Test (i686-linux)	1m 4s
Test use-libc (i686-linux)	43s
Test (aarch64-linux)	1m 6s
Test use-libc (aarch64-linux)	58s
Test (powerpc64le-linux)	1m 3s
Test use-libc (powerpc64le-linux)	59s
Test (riscv64-linux)	1m 4s
Test use-libc (riscv64-linux)	58s
Test (s390x-linux)	1m 2s
Test (arm-linux)	1m 4s
Test use-libc (arm-linux)	59s
Test (ubuntu-stable)	50s
Test (i686-linux-stable)	51s
Test (aarch64-linux-stable)	1m 4s
Test (riscv64-linux-stable)	57s
Test (s390x-linux-stable)	1m 3s
Test (powerpc64le-linux-stable)	1m 1s
Test (arm-linux-stable)	1m 3s
Test (ubuntu-1.63)	1m 2s
Test (i686-linux-1.63)	1m 5s
Test (aarch64-linux-1.63)	1m 6s
Test (riscv64-linux-1.63)	1m 4s
Test (s390x-linux-1.63)	1m 4s
Test (powerpc64le-linux-1.63)	1m 1s
Test (arm-linux-1.63)	1m 6s
Test (macos-latest)	54s
Test (macos-11)	51s
Test (windows)	45s
Test (windows-2019)	47s
Test (musl)	50s
	47m 8s

Provide feedback