Switch usage from os.*File.Readdir to os.*File.ReadDir.

[fmarmol]

I suggest theses changes because I could see some improvements on my tests regarding the following benchmark:

import (
        "testing"

        "github.com/shirou/gopsutil/v3/net"
)

func BenchmarkConnections(b *testing.B) {
        for i := 0; i < b.N; i++ {
                net.Connections("all")
        }
}

✦ ➜ benchstat old.txt new.txt
name           old time/op    new time/op    delta
Connections-8    20.8ms ± 0%    16.7ms ± 0%   ~     (p=1.000 n=1+1)

name           old alloc/op   new alloc/op   delta
Connections-8    4.38MB ± 0%    4.01MB ± 0%   ~     (p=1.000 n=1+1)

name           old allocs/op  new allocs/op  delta
Connections-8     45.2k ± 0%     42.4k ± 0%   ~     (p=1.000 n=1+1)

Can you confirm that these improvements are relevants?

[Lomanic]

This was introduced in go1.16, while the v3 package minimum version is 1.15.

[fmarmol]

@Lomanic Ok thx for the reply, I didn't pay attention about the version. I think we can close this PR.

[shirou]

Thank you for your contribution!
Go 1.15 will be out of support when 1.17 is released. And perhaps 1.17 will be release within a few month, I want to leave this PR open.

[shirou]

Sorry to late response. Time passed from 1.17 released, I think it's safe to merge now. Thank you for your contribution!