SI-8410 Don't warn fatally on disabled flag

Since boolean settings can now be set false by user,
summary warnings should not be issued when the flag
is explicitly off (as opposed to unset, default).

In particular, `-Xfatal-warnings` should not fail
if there were no warnings otherwise.

```
$ ~/scala-2.11.2/bin/scalac -d /tmp -deprecation:false test/files/pos/t8410.scala
$ ~/scala-2.11.2/bin/scalac -d /tmp -deprecation:false -Xfatal-warnings test/files/pos/t8410.scala
warning: there were three deprecation warnings; re-run with -deprecation for details
error: No warnings can be incurred under -Xfatal-warnings.
one warning found
one error found

```
After this commit:
```
$ skalac -d /tmp -Xfatal-warnings test/files/pos/t8410.scala
warning: there were three deprecation warnings; re-run with -deprecation for details
error: No warnings can be incurred under -Xfatal-warnings.
one warning found
one error found
$ skalac -d /tmp -deprecation:false -Xfatal-warnings test/files/pos/t8410.scala
```
Similarly for other collecting flags:
```
$ skalac -d /tmp -optimise -Yinline-warnings -deprecation:false -Xfatal-warnings test/files/pos/t8410.scala
test/files/pos/t8410.scala:14: warning: Could not inline required method dropWhile because access level required by callee not matched by caller.
  def k = List(0).dropWhile(_ < 1)   // inlining warns doubly
                           ^
test/files/pos/t8410.scala:14: warning: At the end of the day, could not inline @inline-marked method dropWhile
  def k = List(0).dropWhile(_ < 1)   // inlining warns doubly
                           ^
error: No warnings can be incurred under -Xfatal-warnings.
two warnings found
one error found
$ skalac -d /tmp -optimise -Yinline-warnings:false -deprecation:false -Xfatal-warnings test/files/pos/t8410.scala

```
Footnote: handling of deprecated locals also changed in 2014:
```
$ ~/scala-2.11.0-M7/bin/scalac -d /tmp -deprecation -Xfatal-warnings test/files/pos/t8410.scala
test/files/pos/t8410.scala:8: warning: method f in object Test is deprecated:
  Console println f  // warns
                  ^
error: No warnings can be incurred under -Xfatal-warnings.
one warning found
one error found
$ ~/scala-2.11.0-M8/bin/scalac -d /tmp -deprecation -Xfatal-warnings test/files/pos/t8410.scala
test/files/pos/t8410.scala:5: warning: method _f is deprecated:
  def g = { @deprecated("","") def _f = f ; _f }                   // warns in 2.11.0-M8
                                            ^
test/files/pos/t8410.scala:6: warning: class X is deprecated:
  def x = { @deprecated("","") class X { def x = f } ; new X().x } // warns in 2.11.0-M8
                                                           ^
test/files/pos/t8410.scala:8: warning: method f in object Test is deprecated:
  Console println f  // warns
                  ^
error: No warnings can be incurred under -Xfatal-warnings.
three warnings found
one error found

```
SI-8410 Summarize if warnings and not disabled

Simplify the conditional and emit the summary if
there were buffered warnings and the user didn't
explicitly disable this setting.

The working assumption is that using fatal-warnings
to turn on summaries was either an outdated heuristic
or just due to a faulty merge. Or wait, was it for
-feature, which is not enabled when warnings are
issued against it?
Merge pull request #3955 from som-snytt/issue/8410

SI-8410 Don't warn fatally on disabled flag