Give warnings for unknown Doxygen commands in Doxygen parser.

Silently ignoring unknown Doxygen commands is not a reasonable default
behaviour, it's simple enough to turn off the warning if the command is really
supposed to be just ignored, but it's too easy to not notice a real problem if
it isn't.

Turn WARN_DOXYGEN_UNKNOWN_COMMAND on by default and add a test to the errors
test suite checking that it is indeed given.

Examples/test-suite/errors/doxygen_unknown_command.i

@@ -0,0 +1,6 @@
+%module xxx
+
+/**
+    There is an \unknown Doxygen comment here.
+ */
+void foo();