idna: update to Unicode 11

Also adopt Unicode selection based on the Unicode
version used by the go binary, as is done in x/text.

Change-Id: I334897e15b2e6fcc6ca175e7eaaa6262ae838f80
Reviewed-on: https://go-review.googlesource.com/c/net/+/172738
Run-TryBot: Marcel van Lohuizen <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: mpvl