Add `shm_open`/`shm_unlink` (POSIX shared memory) #3322

Job	Run time
Check nightly-only targets (nightly)	28s
Check selected Tier 3 platforms (nightly)	4m 42s
Check --no-default-features (nightly)	1m 15s
Check (stable)	8m 30s
Check (nightly)	8m 11s
Rustfmt	9s
Check (1.63)	14m 42s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 41s
Test (ubuntu)	1m 35s
Test use-libc (ubuntu)	1m 8s
Test (ubuntu-20.04)	1m 2s
Test use-libc (i686-linux)	1m 25s
Test (i686-linux)	1m 23s
Test use-libc (aarch64-linux)	1m 29s
Test (aarch64-linux)	1m 56s
Test use-libc (powerpc64le-linux)	1m 31s
Test (powerpc64le-linux)	2m 4s
Test use-libc (riscv64-linux)	1m 47s
Test (riscv64-linux)	1m 37s
Test use-libc (arm-linux)	1m 48s
Test (s390x-linux)	1m 57s
Test (arm-linux)	1m 31s
Test (ubuntu-stable)	1m 17s
Test (i686-linux-stable)	1m 32s
Test (aarch64-linux-stable)	1m 57s
Test (riscv64-linux-stable)	1m 27s
Test (s390x-linux-stable)	2m 11s
Test (powerpc64le-linux-stable)	1m 46s
Test (arm-linux-stable)	1m 48s
Test (ubuntu-1.63)	5m 44s
Test (i686-linux-1.63)	4m 52s
Test (aarch64-linux-1.63)	5m 10s
Test (riscv64-linux-1.63)	4m 37s
Test (s390x-linux-1.63)	6m 4s
Test (powerpc64le-linux-1.63)	5m 37s
Test (arm-linux-1.63)	4m 48s
Test (macos-latest)	1m 17s
Test (macos-11)	1m 12s
Test (windows)	3m 39s
Test (windows-2019)	1m 16s
	1h 58m 5s

Provide feedback