version/cmdname: s/path/filepath/ and fix version.ReadExe() fallback.

We were using the Go 'path' module, which apparently doesn't handle
backslashes correctly. path/filepath does.

However, the main bug turned out to be that we were not calling .Base()
on the path if version.ReadExe() fails, which it seems to do at least
on Windows 7. As a result, our logfile persistence was not working on
Windows, and logids would be regenerated on every restart.

Affects: #620

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

version/cmdname.go

@@ -8,7 +8,7 @@ package version
 
 import (
 	"os"
-	"path"
+	"path/filepath"
 	"strings"
 
 	"rsc.io/goversion/version"
@@ -26,12 +26,13 @@ func CmdName() string {
 	v, err := version.ReadExe(e)
 	if err != nil {
 		ret = strings.TrimSuffix(strings.ToLower(e), ".exe")
+		ret = filepath.Base(ret)
 	} else {
 		// v is like:
 		// "path\ttailscale.com/cmd/tailscale\nmod\ttailscale.com\t(devel)\t\ndep\tgithub.com/apenwarr/fixconsole\tv0.0.0-20191012055117-5a9f6489cc29\th1:muXWUcay7DDy1/hEQWrYlBy+g0EuwT70sBHg65SeUc4=\ndep\tgithub....
 		for _, line := range strings.Split(v.ModuleInfo, "\n") {
 			if strings.HasPrefix(line, "path\t") {
-				ret = path.Base(strings.TrimPrefix(line, "path\t"))
+				ret = filepath.Base(strings.TrimPrefix(line, "path\t"))
 				break
 			}
 		}