command line: read-only databases and --print-databases

Read-only database are shown with a new "<read-only>" tag.
There is no explicit read/write flag, so a lack of that tag
is no guarantee that a database is really writable. It depends
on the backend whether it checks for write acccess.

src/syncevo/Cmdline.cpp

@@ -2072,6 +2072,9 @@ void Cmdline::listDatabases(SyncSource *source, const string &header)
             if (database.m_isDefault) {
                 out << " <default>";
             }
+            if (database.m_isReadOnly) {
+                out << " <read-only>";
+            }
             out << endl;
         }
     }