Do not cache boot time for linux. Fix #837

[Gui13]

This should fix the issue #837: we now query the value of /proc/stat or /proc/uptime at each invocation.

Not doing so will not return the correct boot time when NTP adjusts the time of the system in linux.

[Lomanic]

Could you test this with and without your patch applied please?

1. stop the NTP service
2. Reboot so that time is wrong on the Raspberry Pi
3. Launch this program below, before ntpd kicks in
4. Launch ntpd, time jumps
5. Observe what's reported by this test program (is it still reported as running after time jumps?)

package main

import (
        "fmt"
        "os"
        "time"

        "github.com/shirou/gopsutil/process"
)

func main() {
        p, _ := process.NewProcess(int32(os.Getpid()))

        c := time.Tick(5 * time.Second)
        for range c {
                fmt.Printf("%s ", time.Now())
                fmt.Println(p.IsRunning())
        }
}

[Lomanic]

I was able to test my program above on my Rapsberry Pi and IsRunning() is returning false for the current process after time jumps, with or without this patch. I didn't test host.Uptime() though, but it's probably good. Will test it soon and merge if OK. Thanks for your patience.

[Lomanic]

We should use unix.Sysinfo in the future to get uptime on Linux.