Add an API for shared memory APIs

[notgull]

softbuffer currently uses shmget, shmctl, and other shared memory APIs. It's the only thing left preventing us from switching from nix to rustix.

[sunfishcode]

At a glance, the specific items needed seem to be:

• shmat
• shmdt
• shmget
• shmctl
• ShmAtFlags bitflags for SHM_* for shmat
• ShmGetFlags bitflags for SHM_* and IPC_* for shmget
• ShmIdDs type to correspond to struct shmid_ds
• ShmCmd enum for the SHM_* and IPC_* that can be used with shmctl