net: use libresolv rules for ndots range and validation

BIND libresolv allows values from 0 to 15.

For invalid values and negative numbers, 0 is used.
For numbers greater than 15, 15 is used.

Fixes #15419

Change-Id: I1009bc119c3e87919bcb55a80a35532e9fc3ba52
Reviewed-on: https://go-review.googlesource.com/24901


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>