Replace `EOK` with `0`

[linguini1]

Turns out that EOK is a QNX-specific macro (which is crazy to me, but POSIX defines literally every other error type in an enum of E* names except EOK as 0).

To properly be POSIX compatible, we should either:

• Replace occurrences of EOK with 0
• Add a #define EOK 0 somewhere in a common header file so that our code will compile elsewhere

I do like the readability of EOK