MXS-2008 Provide single entrypoint for initializing maxbase

Everything of maxbase can now be initialized by a call to

    maxbase_init();

(from a C-program) or

    maxbase::init();

from a C++-program and finalized with calls to either
maxbase_finish() or maxbase::finish(). Creating an instance
maxbase::MaxBase will take care of both operations.

maxutils/maxbase/src/log.cc

@@ -500,6 +500,11 @@ void mxb_log_finish(void)
     this_unit.context_provider = nullptr;
 }
 
+bool mxb_log_inited()
+{
+    return this_unit.sLogger && this_unit.sMessage_registry;
+}
+
 void mxb_log_set_augmentation(int bits)
 {
     this_unit.augmentation = bits & MXB_LOG_AUGMENTATION_MASK;