MEDIUM: proxy: use global proxy list for REGISTER_POST_PROXY_CHECK() hook

MEDIUM: proxy: use global proxy list for REGISTER_POST_PROXY_CHECK() hook

REGISTER_POST_PROXY_CHECK() used to iterate over "main" proxies to run
registered callbacks. This means hidden proxies (and their servers) did
not get a chance to get post-checked and could cause issues if some post-
checks are expected to be executed on all proxies no matter their type.

Instead we now rely on the global proxies list. Another side effect is that
the REGISTER_POST_SERVER_CHECK() now runs as well for servers from proxies
that are not part of the main proxies list.