acme: Make sure we don't handle the same listener twice
If a daemon listens on multiple addresses at once, it'll show up multiple
times in get_listeners() which will clobber the config for uhttpd. Fix this
by skipping subsequent handlings of the same daemon binary.
Fixes#13325.
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>