Add System V shared memory APIs

[arpankapoor]

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First draft to add System V shared memory APIs. I would like some help with these questions:

1. How to decide if an exported function should be unsafe or not? For now, I've marked functions taking in any pointer arguments as unsafe.
2. Should we have libc_bitflags for shmflg or cmd? There aren't a set of constants with a fixed prefix as described in the CONVENTIONS.md file. There are a few IPC_* and SHM_* constants for them.

Partially addresses #1718.