Toning down the noise of -Xlint and allow `-optimise -Yinline:false`

Motivation in the commit messages. I think the deluge of warnings
form unused imports is a regression that will turn away X-linters.

And we've had reports of spurios "unused local" warnings, too,
that suggest this feature needs more time to stabilize.

While I was in settings land, I implemented something I've been
missing for ages, the ability to disable individual settings within
a group.