BUG/MINOR: proxy: also look for the closest option among cfg_opts3

BUG/MINOR: proxy: also look for the closest option among cfg_opts3

proxy_find_best_option() scans cfg_opts[] and cfg_opts2[] for the option
which looks closest to a mistyped one. When cfg_opts3[] was introduced
in 3.2 with commit cc5a66212d ("MINOR: proxy: add proxy->options3"), it
was not added there, so such newly added options are not proposed as
candidates for mistakes (e.g. "assume-rfc6587-ntf" or "dont-parse-log").
Let's just fix this.

This may be backported to 3.2.