MEDIUM: reload: pass all exportable FDs, not just listeners

MEDIUM: reload: pass all exportable FDs, not just listeners

Now we don't limit ourselves to listeners found in proxies nor peers
anymore, we're instead scanning all known FDs for those marked with
".exported=1". Just doing so has significantly simplified the code,
and will later allow to yield while sending FDs if desired.

When it comes to retrieving a possible namespace name or interface
name, for now this is only performed on listeners since these are the
only ones carrying such info. Once this moves somewhere else, we'll
be able to also pass these info for UDP receivers for example, with
only tiny changes.