Add `shm_open`/`shm_unlink` (POSIX shared memory) #3318

Job	Run time
Rustfmt	7s
Check (stable)	3m 31s
Check --no-default-features (nightly)	1m 11s
Check (nightly)	3m 22s
Check nightly-only targets (nightly)	24s
Check (1.63)	3m 38s
Check selected Tier 3 platforms (nightly)	6m 8s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 26s
Test use-libc (ubuntu)	1m 11s
Test (ubuntu)	1m 12s
Test use-libc (i686-linux)	1m 23s
Test (ubuntu-20.04)	1m 2s
Test use-libc (aarch64-linux)	1m 19s
Test (i686-linux)	1m 23s
Test (aarch64-linux)	1m 34s
Test use-libc (powerpc64le-linux)	1m 32s
Test (powerpc64le-linux)	1m 33s
Test use-libc (riscv64-linux)	1m 41s
Test (riscv64-linux)	1m 54s
Test use-libc (arm-linux)	1m 24s
Test (s390x-linux)	1m 28s
Test (arm-linux)	1m 46s
Test (ubuntu-stable)	1m 8s
Test (i686-linux-stable)	1m 28s
Test (aarch64-linux-stable)	1m 25s
Test (riscv64-linux-stable)	1m 52s
Test (s390x-linux-stable)	1m 45s
Test (powerpc64le-linux-stable)	1m 32s
Test (arm-linux-stable)	1m 33s
Test (ubuntu-1.63)	1m 54s
Test (i686-linux-1.63)	1m 55s
Test (aarch64-linux-1.63)	1m 57s
Test (riscv64-linux-1.63)	1m 56s
Test (s390x-linux-1.63)	1m 55s
Test (powerpc64le-linux-1.63)	1m 55s
Test (arm-linux-1.63)	1m 54s
Test (macos-latest)	1m 35s
Test (macos-11)	1m 17s
Test (windows)	1m 36s
Test (windows-2019)	1m 38s
	1h 10m 24s

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