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/util/time.cpp

@@ -7,6 +7,7 @@
 #include <config/bitcoin-config.h>
 #endif
 
+#include <compat.h>
 #include <util/time.h>
 
 #include <atomic>
@@ -114,3 +115,11 @@ int64_t ParseISO8601DateTime(const std::string& str)
         return 0;
     return (ptime - epoch).total_seconds();
 }
+
+struct timeval MillisToTimeval(int64_t nTimeout)
+{
+    struct timeval timeout;
+    timeout.tv_sec  = nTimeout / 1000;
+    timeout.tv_usec = (nTimeout % 1000) * 1000;
+    return timeout;
+}