pkg/config: Rewind *File after writeDefaultConfig

Otherwise the later call to ioutil.ReadAll will return an empty array.

This issue is visible from the user's perspective due to the new
debug-info-directories parameter, which does have a value in the default
config.

As a consequence, the first 'dlv' invocation fails to find the separate
debug info files, while the second one works as expected:

$ ./dlv core /usr/bin/dockerd-current /case/dockerd.1234
could not find external debug info file
$ ./dlv core /usr/bin/dockerd-current /case/dockerd.1234
Type 'help' for list of commands.
(dlv) exit

pkg/config/config.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"os/user"
@@ -129,6 +130,7 @@ func createDefaultConfig(path string) (*os.File, error) {
 	if err != nil {
 		return nil, fmt.Errorf("unable to write default configuration: %v", err)
 	}
+	f.Seek(0, io.SeekStart)
 	return f, nil
 }