Add millisecond accessor to Genode::Duration value object

Add a 'trunc_to_plain_ms' method to Gende::Duration to make
millisecond-accurate timing safer and more convenient.

Ref #2335

repos/base/lib/symbols/ld

@@ -359,6 +359,7 @@ _ZNK6Genode5Child21notify_resource_availEv T
 _ZNK6Genode6Thread10stack_baseEv T
 _ZNK6Genode6Thread4nameEv T
 _ZNK6Genode6Thread9stack_topEv T
+_ZNK6Genode8Duration17trunc_to_plain_msEv T
 _ZNK6Genode8Duration17trunc_to_plain_usEv T
 _ZNK6Genode8Duration9less_thanERS0_ T
 _ZNKSt13bad_exception4whatEv T

repos/os/include/os/duration.h

@@ -80,6 +80,7 @@ struct Genode::Duration
 		explicit Duration(Microseconds us) { add(us); }
 
 		Microseconds trunc_to_plain_us() const;
+		Milliseconds trunc_to_plain_ms() const;
 };
 
 #endif /* _OS__DURATION_H_ */

repos/os/src/lib/timeout/duration.cc

@@ -82,3 +82,9 @@ Microseconds Duration::trunc_to_plain_us() const
 {
 	return Microseconds(_microseconds + (_hours ? _hours * US_PER_HOUR : 0));
 }
+
+
+Milliseconds Duration::trunc_to_plain_ms() const
+{
+	return Milliseconds(trunc_to_plain_us().value / 1000);
+}