some spelling corrections

The pull request included some changes that were neither accidental nor
unintentional, so only a subset of the changes from pull request #869
submitted by klorpa were manually applied.

behaviour  -> behavior
speach     -> speech
knowlege   -> knowledge
incrments  -> increments
stethscope -> stethoscope
staiway    -> stairway
arifact    -> artifact
extracing  -> extracting

The uses of "iff" were left alone.

Close #869

outdated/win/Qt3/qt3_win.cpp

@@ -4283,7 +4283,7 @@ char NetHackQtYnDialog::Exec()
 }
 void NetHackQtYnDialog::keyPressEvent(QKeyEvent* event)
 {
-    // Don't want QDialog's Return/Esc behaviour
+    // Don't want QDialog's Return/Esc behavior
     event->ignore();
 }