unify API - all stats grabbers now output entries

In former times, only those stats grabbers which return (likely) multiple
items had an optional size_t output argument "entries". Meanwhile, it would
be possible to grab cpu stats per CPU, load stats per CPU, swap stats per
swap device etc.

Even if the content of those stats might be worth being discussed,
breaking API multiple times isn't recommended. With coming release, most
developers have to fix their callers anyway (from int * => size_t *),
having some additional fix points doesn't bother more.