SYSV Shared Memory functions (shmget/shmat/shmdt/shmctl)

[rennergade]

We'll need to add back the SYSV shared memory functions to get postgres running eventually.

I believe the way to do this simply is to creating mapping helper functions similar to the mmap_helper that Qianxi added in the vmmap PR, and then combine those with the segment management infrastructure from RustPOSIX. The syscalls will then look largely the same as before.

Opening this issue to get feedback before Chinmay starts working on this next week.