SI-7707 SI-7712 Exclude unused warnings from -Xlint

Experience building open source projects like Specs that use
`-Xlint` suggests that this warning is too noisy to lump in
with the others.

We are lacking in more fine-grained control of these things, so
simply turning of `-Xlint` in favour of its underlying `-Y` options
ends up *losing* some other important warnings that are predicated
directly on `-Xlint`.

Furthermore, bug reports against M8, SI-7707 SI-7712, show that
unused private/local warnings, while far less noisy, are still
in need of polish.

This commit moves these warnings to a pair of new -Y options,
neither of which is part of  `-Xlint`..

Let's ask people to opt in for 2.11, and as it stabilizes, we can
consider adding it to Xlint (or the desirable evolution of that)
in the next release.
Group settings should honor user-set individual options

In the last commit, we added the ability to explicitly disable
boolean settings form the command line.

This commit changes group settings (like -optimize), to leave
them alone if set explicitly.

Examples:

   `scalac -Xlint -Ywarn-unused:false -optimize -Yinline:false`

The mechanism for such settings has also been refactored to
`MutableSettings`, to let use reuse this for `-Xlint`.