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

Summary:
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.

This is a backport of [[https://github.com/bitcoin/bitcoin/pull/20788 | core#20788]] [1/5]
https://github.com/bitcoin/bitcoin/pull/20788/commits/aa17a44551c03b00a47854438afe9f2f89b6ea74

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11003