Use OS calls rather than exec() to get memory stats on Darwin

[walles]

Before this change we used to exec() various binaries to find out memory
information.

While this worked, it was awfully slow.

And if somebody would want to compute how many percent of available memory all
PIDs on the system uses, that would take almost ten seconds on my laptop with
the previous implementation.

This implementation fares a lot better, and is smaller.

[shirou]

At first, I wanted about avoiding cgo, so current implementation uses exec. But from #66, cgo is introduced to cpu, so we cross the Rubicon.

I can merge your series of excellent PRs, but could you move (not delete) current implementation to no_cgo file like cpu/cpu_darwin_nocgo.go, with !cgo build option? It can give options about use cgo or not to users.

[walles]

CGO / non-CGO split done.

[shirou]

Thanks a lot! CI also passed.