Exit should be constructed explicitly

Co-authored-by: pennae <[email protected]>

src/libutil/exit.hh

@@ -12,7 +12,7 @@ class Exit : public std::exception
 public:
     int status;
     Exit() : status(0) { }
-    Exit(int status) : status(status) { }
+    explicit Exit(int status) : status(status) { }
     virtual ~Exit();
 };