{net,os/user,plugin}: eliminate unnecessary C round-trips

{net,os/user,plugin}: eliminate unnecessary C round-trips

We're making two extra round-trips to C to malloc and free strings
that originate in Go and don't escape. Skip those round-trips by
allocating null-terminated slices in Go memory instead.

Change-Id: I9e4c5ad999a7924ba50b82293c52073ec75518be
Reviewed-on: https://go-review.googlesource.com/56530
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>