A cleanup for MDEV-16939: avoid timeval initialization related problems in the future (compilation failures on Windows)

Adding a helper class Timeval, to initialize "struct timeval" in a safe way.
As a bonus, adding a  method Timeval::trunc(), so the caller now can have
one line instead of five lines (declaration, initializations of sec and usec,
truncation, passing to store_TIMEVAL()).

sql/structs.h

@@ -831,4 +831,20 @@ public:
 };
 
 
+class Timeval: public timeval
+{
+public:
+  Timeval(my_time_t sec, ulong usec)
+  {
+    tv_sec= sec;
+    tv_usec= usec;
+  }
+  Timeval &trunc(uint dec)
+  {
+    my_timeval_trunc(this, dec);
+    return *this;
+  }
+};
+
+
 #endif /* STRUCTS_INCLUDED */