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/netbase.h

@@ -62,10 +62,6 @@ bool CloseSocket(SOCKET& hSocket);
 bool SetSocketNonBlocking(const SOCKET& hSocket, bool fNonBlocking);
 /** Set the TCP_NODELAY flag on a socket */
 bool SetSocketNoDelay(const SOCKET& hSocket);
-/**
- * Convert milliseconds to a struct timeval for e.g. select.
- */
-struct timeval MillisToTimeval(int64_t nTimeout);
 void InterruptSocks5(bool interrupt);
 
 #endif // BITCOIN_NETBASE_H