Add `shm_open`/`shm_unlink` (POSIX shared memory) #3319

Job	Run time
Check --no-default-features (nightly)	1m 27s
Check nightly-only targets (nightly)	31s
Rustfmt	6s
Check (stable)	8m 9s
Check (nightly)	7m 59s
Test use-libc (ubuntu)	59s
Check (1.63)	15m 50s
Test (ubuntu)	1m 9s
Test use-libc (i686-linux)	1m 48s
Test use-libc (aarch64-linux)	1m 52s
Test (ubuntu-20.04)	1m 5s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 29s
Test use-libc (powerpc64le-linux)	1m 27s
Test use-libc (riscv64-linux)	1m 57s
Test (i686-linux)	1m 21s
Test use-libc (arm-linux)	1m 50s
Test (aarch64-linux)	1m 24s
Check selected Tier 3 platforms (nightly)	4m 35s
Test (powerpc64le-linux)	1m 23s
Test (riscv64-linux)	1m 22s
Test (s390x-linux)	1m 21s
Test (arm-linux)	1m 23s
Test (ubuntu-stable)	58s
Test (i686-linux-stable)	1m 16s
Test (aarch64-linux-stable)	1m 23s
Test (riscv64-linux-stable)	1m 19s
Test (s390x-linux-stable)	1m 22s
Test (powerpc64le-linux-stable)	1m 21s
Test (arm-linux-stable)	1m 23s
Test (ubuntu-1.63)	1m 22s
Test (i686-linux-1.63)	1m 25s
Test (aarch64-linux-1.63)	1m 22s
Test (riscv64-linux-1.63)	1m 22s
Test (s390x-linux-1.63)	1m 22s
Test (powerpc64le-linux-1.63)	1m 22s
Test (arm-linux-1.63)	1m 24s
Test (macos-latest)	6m 4s
Test (macos-11)	6m 3s
Test (windows)	1m 4s
Test (windows-2019)	1m 8s
	1h 33m 27s

Provide feedback