Handle numeric DS names

Hi,

I just made a change in order to handle fully numeric DS names in RRD entries. By example, having:

```
ds[42].index = 0
ds[42].type = "DERIVE"
ds[42].minimal_heartbeat = 600
ds[42].min = 0.0000000000e+00
ds[42].max = NaN
ds[42].last_ds = "57447256"
ds[42].value = 1.4651162791e+01
ds[42].unknown_sec = 0
```
currently fills `ds.index` map with a lot of `[]interface{}(nil)`.

With the patch, we verify that `kname` doesn't start with `ds.`, avoiding the `Atoi` conversion for `ds.*` entries and keeping the `-1` key id.

Let me know what do you think of this patch.

Regards,