MINOR: protocol: add a pair of check_events/ignore_events functions at the ctrl layer

MINOR: protocol: add a pair of check_events/ignore_events functions at the ctrl layer

Right now the connection subscribe/unsubscribe code needs to manipulate
FDs, which is not compatible with QUIC. In practice what we need there
is to be able to either subscribe or wake up depending on readiness at
the moment of subscription.

This commit introduces two new functions at the control layer, which are
provided by the socket code, to check for FD readiness or subscribe to it
at the control layer. For now it's not used.