move-only: Move settings to the common library

The background of this commit is an ongoing effort to decouple the
libbitcoinkernel library from code that is not strictly required by it.
The settings code belongs into the common library and namespace, since
the kernel library should not depend on it. See doc/design/libraries.md
for more information on this rationale.

Changing the namespace of the moved functions is scripted in the
following commit.

src/qt/test/optiontests.h

@@ -5,9 +5,9 @@
 #ifndef BITCOIN_QT_TEST_OPTIONTESTS_H
 #define BITCOIN_QT_TEST_OPTIONTESTS_H
 
+#include <common/settings.h>
 #include <qt/optionsmodel.h>
 #include <univalue.h>
-#include <util/settings.h>
 
 #include <QObject>