net: remove dependency on math/rand

Like we did for sync, let the runtime give net random numbers,
to avoid forcing an import of math/rand for DNS.

Change-Id: Iab3e64121d687d288a3961a8ccbcebe589047253
Reviewed-on: https://go-review.googlesource.com/c/go/+/241258


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>