BUG/MEDIUM: sink: fix unexpected double postinit of sink backend

BUG/MEDIUM: sink: fix unexpected double postinit of sink backend

Willy experienced an unexpected behavior with the config below:

    global
        stats socket :1514

    ring buf1
        server srv1 127.0.0.1:1514

Indeed, haproxy would connect to the ring server twice since commit 23e5f18b
("MEDIUM: sink: change the sink mode type to PR_MODE_SYSLOG"), and one of the
connection would report errors.

The reason behind is is, despite the above commit saying no change of behavior
is expected, with the sink forward_px proxy now being set with PR_MODE_SYSLOG,
postcheck_log_backend() was being automatically executed in addition to the
manual cfg_post_parse_ring() function for each "ring" section. The consequence
is that sink_finalize() was called twice for a given "ring" section, which
means the connection init would be triggered twice.. which in turn resulted in
the behavior described above, plus possible unexpected side-effects.

To fix the issue, when we create the forward_px proxy, we now set the
PR_CAP_INT capability on it to tell haproxy not to automatically manage the
proxy (ie: to skip the automatic log backend postinit), because we are about
to manually manage the proxy from the sink API.

No backport needed, this bug is specific to 3.3