BUG/MINOR: proxy: always initialize the trash in show servers state

BUG/MINOR: proxy: always initialize the trash in show servers state

Actually the cleanup in commit 6ff8143f7 ("BUG/MINOR: proxy: fix
dump_server_state()'s misuse of the trash") allowed to spot that the
trash is never reset when dumping a servers state. I couldn't manage
to make it dump garbage even with large setups but didn't find either
where it's cleared between successive calls while other handlers do
explicitly invoke chunk_reset(), so it seems to happen a bit by luck.

Let's use chunk_printf() here for each turn, it makes things clearer.

This could be backported along with previous patch, especially if any
user reports occasional garbage appearing in the show servers output.