SI-8736 Restore -language to former glory

MultiChoice allows -language to work like -Xlint.

The bug as described was that the setting value was set instead of updated
(++=) with additional flags.

The unreported bug was that `_` no longer set all settings.

The corrected behavior is that "contains" means "it was enabled, or
_ was set and it was not disabled explicitly."

That is the behavior of `-Xlint` but with a different mechanism,
since each lint option is a Setting.

A semantic difference is that -Xlint enables all the lint options,
but -language does not enable all the language features. `scalac -X` does
not explain this additional behavior of the `-Xlint` flag.

Also worth noting that `scalac -feature -language unused.scala` failed
in 2.11.1 but succeeds silently now.
SI-8736 Error if no arg and no default

Now `-language` with no option will emit an error:

```
$ skalac -feature -language unused.scala
scalac error: '-language' requires an option. See '-language:help'.
  scalac -help  gives more information
```
Merge pull request #3888 from som-snytt/issue/8736

SI-8736 Restore -language to former glory