backend: let ParseConfig return a Config pointer

In order to change the backend initialization in `global.go` to be able
to generically call cfg.ApplyEnvironment() for supported backends, the
`interface{}` returned by `ParseConfig` must contain a pointer to the
configuration.

An alternative would be to use reflection to convert the type from
`interface{}(Config)` to `interface{}(*Config)` (from value to pointer
type). However, this would just complicate the type mess further.

internal/backend/local/config.go

@@ -27,12 +27,12 @@ func init() {
 }
 
 // ParseConfig parses a local backend config.
-func ParseConfig(s string) (Config, error) {
+func ParseConfig(s string) (*Config, error) {
 	if !strings.HasPrefix(s, "local:") {
-		return Config{}, errors.New(`invalid format, prefix "local" not found`)
+		return nil, errors.New(`invalid format, prefix "local" not found`)
 	}
 
 	cfg := NewConfig()
 	cfg.Path = s[6:]
-	return cfg, nil
+	return &cfg, nil
 }