syscall: port fix for netbsd unix sockets from upstream
syscall: port fix for netbsd unix sockets from upstream

NetBSD does not include the null terminator when in its reported socket
length. Port the upstream bugfix for the issue (#6627).

This was likely missed during the usual upstream merge because the gc
and gccgo socket implementations have diverged quite a bit.

Change-Id: Id808bae481b5de494cfc641ca9d7f56fef6acd41
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/261741
Trust: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend