cmd/go: a raft of fixes

* add -work option to save temporary files (Fixes issue 2980)
* fix go test -i to work with cgo packages (Fixes issue 2936)
* do not overwrite/remove empty directories or non-object
  files during build (Fixes issue 2829)
* remove package main vs package non-main heuristic:
  a directory must contain only one package (Fixes issue 2864)
* to make last item workable, ignore +build tags for files
  named on command line: go build x.go builds x.go even
  if it says // +build ignore.
* add // +build ignore tags to helper programs

R=golang-dev, r, r
CC=golang-dev
https://golang.org/cl/5674043

generate_cert.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // Generate a self-signed X.509 certificate for a TLS server. Outputs to
 // 'cert.pem' and 'key.pem' and will overwrite existing files.