Storage OpenBSD/adJ

[vtamara]

Why this should be merged

Starts to add support for OpenBSD/adJ as required by #2782

How this works

Using build constraints and the right field names for syscall.Statfs_t in OpenBSD as described at golang/go#47958

How this was tested

With the following storage_openbsd_test.go:

package storage

import (
    //"fmt"
    "os/exec"
    "strconv"
    "strings"
    "testing"
)


func TestAvalableBytes(t *testing.T) {
	avail1, err := AvailableBytes("/home");
	if avail1 <= 0 || err != nil {
		t.Fatalf(`avail1 = %d, err = %v`, avail1, err.Error())
	}

	stdout, err := exec.Command("/bin/df", "/home").Output()
	// In OpenBSD the df program returns something like:
	// df /home
	// Filesystem  512-blocks      Used     Avail Capacity  Mounted on
	// /dev/sd0g     75663352  64674648   7205540    90%    /home  
	if err != nil {
		t.Fatalf(`err = %v`, err.Error())
	}
	//fmt.Printf("output %s\n", stdout)
	f := strings.Fields(string(stdout));
	bavail2, err := strconv.ParseUint(f[10], 10, 64)
	if err != nil {
		t.Fatalf(`err = %v`, err.Error())
	}
	avail2 := bavail2 * 512;

	if avail1 != avail2 {
		t.Fatalf(`avail1 = %d, avail2 = %d`, avail1, avail2)
	}

}