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 `_`.)
SI-8616 Error on -deprecation:true,false

This is an error, not a crash.

Alternatively, one could define the multivalued colon case
as equivalent to specifying the option multiple times. That
would be very regular.  But sometimes it's nicer just to error out.
SI-8525 Add -Xlint:-warn-missing-interpolator

Turn off lint warnings with negating prefix, and add a lint-only
warning for the infamously nagging "Did you forget the interpolator?"

That message is made more dignified.

Without `-Xlint:false`, there is no mechanism to turn off
anonymous linters once `-Xlint` is selected.
SI-8525 No anonymous lint

Turn anonymous references to `settings.lint` into named settings.

After that, trust to Adriaan to make them filterable.

There are a few remaining top-level -Y lint warnings that are
deprecated.
Merge pull request #3792 from som-snytt/issue/8525

SI-8525 -Xlint:nowarn-missing-interpolator