net: don't ignore getsockname errors in newFileFD

net: don't ignore getsockname errors in newFileFD

newFileFD is called when creating a net FD from an existing socket
handle. That socket might not be bound yet, in which case getsockname
returns a useful error that is currently ignored and replaced with a
potentially misleading EPROTONOSUPPORT error later on.

Updates #73696
Updates #74976
Updates #75282
Updates #75279
Updates #76537
Updates #76582
Updates #77038

Change-Id: I2a8b30ffbb037035669f65a95a923edc8b288145
Reviewed-on: https://go-review.googlesource.com/c/go/+/734820
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Damien Neil <dneil@google.com>