MINOR: proxy: Be able to reference the defaults section used by a proxy

MINOR: proxy: Be able to reference the defaults section used by a proxy

A proxy may now references the defaults section it is used. To do so, a
pointer on the default proxy was added in the proxy structure. And a
refcount must be used to track proxies using a default proxy. A default
proxy is destroyed iff its refcount is equal to zero and when it drops to
zero.

All this stuff must be performed during init/deinit staged for now. All
unreferenced default proxies are removed after the configuration parsing.

This patch is mandatory to support TCP/HTTP rules in defaults sections.