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

Also 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. (There is no all option yet here.)

-Xlint and -Xlint:_ just set a flag which is consulted by any
unset lint warning.

Xlint warnings consult the state of Xlint when they are unset.

Individual -Ywarn-ings do not.

Other warnings are explicitly set to false. They can only be
enabled programmatically.

Some tests are corrected. Also, option order is no longer
significant, see the unit test.