improve alpha sort in mflag

```
go run example.go -h
  --integer, --number=-1: a simple integer
  -b=false: a simple bool
  -h, --help=false: display the help
  -s, --string="": a simple string
```

becomes:

```
o run example.go -h
  -b=false: a simple bool
  -h, --help=false: display the help
  --integer, --number=-1: a simple integer
  -s, --string="": a simple string
```


will be useful in #4174 (@vbatts) and  #4441 (@crosbymichael)