SI-8525 Clarify usage of -Xlint:_,flag

Align more with javac -Xlint:all,-flag,flag where once a flag is
explicitly enabled it cannot be disabled, but where the wildcard
is a backstop only.

-Xlint and -Xlint:_ just set a flag which is consulted by any
unset lint warning.

Some tests are corrected. Also, option order is no longer
significant, see the unit test.