BUG/MEDIUM: mux-fcgi: Don't rely on SI src/dst addresses for FCGI health-checks

BUG/MEDIUM: mux-fcgi: Don't rely on SI src/dst addresses for FCGI health-checks

When an HTTP health-check is performed in FCGI, we must not rely on the SI
source and destination addresses to set default parameters
(REMOTE_ADDR/REMOTE_PORT and SERVER_NAME/SERVER_PORT) because the backend
conn-stream is not attached to a stream but to a healt-check. Thus, there is
no stream-interface. In addition, there is no client connection because it
is an "internal" session.

Thus, for now, in this case, there is only the server connection that can be
used. So src/dst addresses are retrieved from the server connection when the
CS application is a health-check.

This patch should solve issue #1572. It must be backported to 2.5. Note than
the CS api has changed. Thus, on HAProxy 2.5, we should test the session's
origin instead:

    const struct sockaddr_storage *src = (cs_check(fstrm->cs) ? ...);
    const struct sockaddr_storage *dst = (cs_check(fstrm->cs) ? ...);