Add `shm_open`/`shm_unlink` (POSIX shared memory) #3279

Job	Run time
Check (stable)	3m 24s
Check (nightly)	3m 10s
Check selected Tier 3 platforms (nightly)	3m 28s
Check (1.63)	3m 28s
Check --no-default-features (nightly)	1m 37s
Check nightly-only targets (nightly)	23s
Rustfmt	16s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 33s
Test use-libc (ubuntu)	1m 5s
Test use-libc (i686-linux)	1m 29s
Test use-libc (aarch64-linux)	1m 31s
Test use-libc (powerpc64le-linux)	2m 10s
Test (ubuntu)	1m 2s
Test use-libc (riscv64-linux)	1m 36s
Test (ubuntu-20.04)	1m 11s
Test use-libc (arm-linux)	1m 38s
Test (i686-linux)	1m 11s
Test (aarch64-linux)	1m 14s
Test (powerpc64le-linux)	1m 12s
Test (riscv64-linux)	1m 12s
Test (s390x-linux)	1m 11s
Test (arm-linux)	1m 13s
Test (ubuntu-stable)	1m 7s
Test (i686-linux-stable)	1m 15s
Test (aarch64-linux-stable)	1m 12s
Test (riscv64-linux-stable)	1m 12s
Test (s390x-linux-stable)	1m 11s
Test (powerpc64le-linux-stable)	1m 12s
Test (arm-linux-stable)	1m 14s
Test (ubuntu-1.63)	1m 10s
Test (i686-linux-1.63)	1m 11s
Test (aarch64-linux-1.63)	1m 14s
Test (riscv64-linux-1.63)	1m 12s
Test (s390x-linux-1.63)	1m 13s
Test (powerpc64le-linux-1.63)	1m 11s
Test (arm-linux-1.63)	1m 15s
Test (macos-latest)	43s
Test (macos-11)	52s
Test (windows)	58s
Test (windows-2019)	1m 1s
	56m 37s

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