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.cpp

@@ -10,6 +10,7 @@
 #include <util/strencodings.h>
 #include <util/string.h>
 #include <util/system.h>
+#include <util/time.h>
 
 #include <atomic>
 #include <cstdint>
@@ -271,14 +272,6 @@ CService LookupNumeric(const std::string& name, int portDefault)
     return addr;
 }
 
-struct timeval MillisToTimeval(int64_t nTimeout)
-{
-    struct timeval timeout;
-    timeout.tv_sec  = nTimeout / 1000;
-    timeout.tv_usec = (nTimeout % 1000) * 1000;
-    return timeout;
-}
-
 /** SOCKS version */
 enum SOCKSVersion: uint8_t {
     SOCKS4 = 0x04,