BUG/MINOR: proxy: add missing interface bind free in free_proxy

BUG/MINOR: proxy: add missing interface bind free in free_proxy

bind->settings.interface hint is allocated when "interface" keyword
is specified on a bind line, but the string isn't explicitly freed in
proxy_free, resulting in minor memory leak on deinit paths when the
keyword is being used.

It partially fixes GH #2173 and may be backported to all stable versions.

[in 2.2 free_proxy did not exist so the patch must be applied directly
in deinit() function from haproxy.c]