sources: add require option

sources: add require option

Require that at least one of the sources specified with this option is
selectable (i.e. recently reachable and not a falseticker) before
updating the clock. Together with the trust option this may be useful to
allow a trusted, but not very precise, reference clock or a trusted
authenticated NTP source to be safely combined with unauthenticated NTP
sources in order to improve the accuracy of the clock. They can be
selected and used for synchronization only if they agree with the
trusted and required source.