Add `shm_open`/`shm_unlink` (POSIX shared memory) (#848) #3328

Job	Run time
Check nightly-only targets (nightly)	32s
Check selected Tier 3 platforms (nightly)	4m 55s
Rustfmt	12s
Check --no-default-features (nightly)	1m 39s
Check (stable)	8m 34s
Check (nightly)	8m 7s
Test rustix_use_experimental_asm (powerpc64le-linux)	2m 9s
Check (1.63)	16m 3s
Test (ubuntu)	1m 4s
Test (ubuntu-20.04)	1m 23s
Test use-libc (ubuntu)	1m 25s
Test (i686-linux)	1m 25s
Test use-libc (i686-linux)	1m 30s
Test (aarch64-linux)	1m 31s
Test use-libc (aarch64-linux)	1m 34s
Test (powerpc64le-linux)	2m 25s
Test use-libc (powerpc64le-linux)	1m 40s
Test (riscv64-linux)	1m 34s
Test use-libc (riscv64-linux)	1m 27s
Test (s390x-linux)	1m 29s
Test use-libc (arm-linux)	1m 28s
Test (arm-linux)	1m 45s
Test (ubuntu-stable)	57s
Test (i686-linux-stable)	1m 19s
Test (aarch64-linux-stable)	1m 28s
Test (riscv64-linux-stable)	1m 25s
Test (s390x-linux-stable)	1m 47s
Test (powerpc64le-linux-stable)	1m 33s
Test (arm-linux-stable)	1m 42s
Test (ubuntu-1.63)	4m 29s
Test (i686-linux-1.63)	4m 48s
Test (aarch64-linux-1.63)	6m 1s
Test (riscv64-linux-1.63)	5m 8s
Test (s390x-linux-1.63)	4m 46s
Test (powerpc64le-linux-1.63)	4m 48s
Test (arm-linux-1.63)	4m 54s
Test (macos-latest)	1m 14s
Test (macos-11)	1m 11s
Test (windows)	1m 40s
Test (windows-2019)	1m 17s
	1h 54m 18s

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