fix build and path error

internal/common/common_darwin.go

@@ -16,7 +16,7 @@ func DoSysctrl(mib string) ([]string, error) {
 		return []string{}, err
 	}
 
-	sysctl, err := exec.LookPath("/usr/bin/getconf")
+	sysctl, err := exec.LookPath("/usr/sbin/sysctl")
 	if err != nil {
 		return []string{}, err
 	}

mem/mem_darwin_nocgo.go

@@ -14,7 +14,7 @@ import (
 func getVMStat(vms *VirtualMemoryStat) error {
 	vm_stat, err := exec.LookPath("vm_stat")
 	if err != nil {
-		return
+		return err
 	}
 	out, err := exec.Command(vm_stat).Output()
 	if err != nil {