SI-8610 -Xlint is multichoice option

Make -Xlint a "multichoice" option for purposes of option parsing.

This allows turning on "lint with these warnings" instead of only
"turn off these warnings but enable other lint warnings".

```
$ scalac -Xlint:warn-adapted-args linty.scala  # lint plus a warning
$ scalac -Xlint warn-adapted-args linty.scala  # same
$ scalac -Xlint linty.scala                    # same as now
$ scalac -Xlint -- linty.scala                 # ok, not necessary
$ scalac -Xlint _ -- linty.scala               # another funky underscore
```

This would also enable Xlint options that are not standalone options,
although that is not implemented in this commit.  For example,
`-Xlint:no-missing-interpolator` could be used to disable that
warning. (There is no `-Xoption:flavor=off` syntax.) (`no-` switches
would not be enabled by `_`.)

Review by @retronym who has expressed opinions about the warning group conundrum