net/http: make triv.go example less insecure
net/http: make triv.go example less insecure

The triv.go example serves the entire contents of $HOME by default.
That seems bad, let's not do that.

Also change it to listen on localhost only.

Change-Id: I8f1b7bd6b7d737852273e2ba82deabc4a2d11f6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/428237
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Tatiana Bradley <ed5904c3174de8861076818d9fdd7f7c949a16e6@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go