child-create: Abort initiating a duplicate CHILD_SA

child-create: Abort initiating a duplicate CHILD_SA

This could happen if an acquire is triggered while we respond to a
CREATE_CHILD_SA request from the peer, or if an acquire is triggered
while an IKE_SA (with its existing CHILD_SAs) is reestablished (also
with break-before-make reauthentication).  Also catches multiple
manual initiations.

Note that this ignores the traffic selectors from acquires (narrowing to
them seems rare in practice anyway).

Duplicates can still get created if e.g. both peers initiate them
concurrently.