MINOR: proxy: store the default target into the frontend's configuration

MINOR: proxy: store the default target into the frontend's configuration

Some services such as peers and CLI pre-set the target applet immediately
during accept(), and for this reason they're forced to have a dedicated
accept() function which does not even properly follow everything the regular
one does (eg: sndbuf/rcvbuf/linger/nodelay are not set, etc).

Let's store the default target when known into the frontend's config so that
it's session_accept() which automatically sets it.