server/admin.c: Don't automatically mark `knock' peers as ephemeral.

server/admin.c: Don't automatically mark `knock' peers as ephemeral.

This was a bad idea on my part.

Consider a server which lives behind NAT, and therefore needs to use
`knock' or some similar dynamic arrangement so as to inform its peers of
its current public address.  This shouldn't be considered ephemeral.  In
particular, if its peer decides, because of flaky networking, say, that
it's no longer available, and sends a `BYE' packet, our server shouldn't
actually stop trying to re-establish the connection.