net: move MillisToTimeval() from netbase to util/time

Move `MillisToTimeval()` from `netbase.{h,cpp}` to
`src/util/system.{h,cpp}`.

This is necessary in order to use `MillisToTimeval()` from a newly
introduced `src/util/sock.{h,cpp}` which cannot depend on netbase
because netbase will depend on it.

src/torcontrol.cpp

@@ -14,6 +14,7 @@
 #include <netbase.h>
 #include <util/strencodings.h>
 #include <util/system.h>
+#include <util/time.h>
 
 #include <vector>
 #include <deque>