Make ancillary files valid markdown
src/cqueues.lua: localise 'unpack'

<= 5.1, `unpack` was in _G.
In 5.2, it was moved to the table library

This change allows operation outside of coroutines in 5.1 again

The `assert` is in the same style as src/socket.lua
Whitespace
src/socket.c: Fix includes

Fix (partially) comments about why each include was needed
Removes limits.h, which was unused