[process][windows] Use win32 API in process.Children() instead of slow WMI call

[Lomanic]

The CreateToolhelp32Snapshot+Process32First+Process32Next combo already
iterates over all processes, so it would be inefficient to enumerate all
processes with process.Processes() and then calling p.Ppid() on each of
them: we just use this combo to get all processes and their ppid in a
single iteration.

This is faster by a factor of 25 compared to the previous WMI call.

Benchmark program:

package main

import (
	"fmt"
	"github.com/shirou/gopsutil/process"
	"os"
	"time"
)

func main() {
	start := time.Now()
	procs, err := process.Processes()
	if err != nil {
		fmt.Println("1", err)
		os.Exit(1)
	}
	for _, p := range procs {
		children, err := p.Children()
		if err != nil {
			fmt.Println("2", err)
			os.Exit(1)
		}
		fmt.Println(p.Pid, children)
	}
	elapsed := time.Now().Sub(start)
	fmt.Println("Elapsed time", elapsed, "(", elapsed/(time.Duration(len(procs))), "per process)")
}

Results:

• before patch (wmi): Elapsed time 6.5693758s ( 56.148511ms per process)
• after patch (win32): Elapsed time 251.0143ms ( 2.127239ms per process)