refactor: use default member initializers

- move constant constructor initializers into in-class default member
  initializers
- clang-tidy modernize-use-default-member-init
  https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-default-member-init.html

fairmq/shmem/Monitor.h

@@ -170,7 +170,7 @@ class Monitor
     bool fInteractive; // running in interactive mode
     bool fViewOnly; // view only mode
     bool fMonitor;
-    bool fSeenOnce; // true is segment has been opened successfully at least once
+    bool fSeenOnce{false};   // true is segment has been opened successfully at least once
     bool fCleanOnExit;
     unsigned int fTimeoutInMS;
     unsigned int fIntervalInMS;