Avoid direct use of libproxy

Avoid direct use of libproxy

Since hexchat already depends on GLib, it's better to use GProxyResolver
instead. This might use libproxy, or not, as appropriate.

P.S. This removes a memory safety issue because proxy_list is allocated
using malloc(), not g_malloc(), and therefore using g_strfreev() is
incorrect. The proper way to free the proxy list returned by libproxy
is to use px_proxy_factory_free_proxies() (but nobody does that because
it was added in libproxy 0.4.16, which is somewhat recent).