Make `get_next()` accept a parameter

Make `get_next()` accept a `processID` as a parameter. `get_next()` now searches
for the next process relative to the given processID instead of the current
ringuffer index position.

Importantly, the only functionw which now affect the ringbuffer index are
`add_process()` and `set_current()`. The codebase now agrees with the
documentation laid out in xrndbuffer.h.