MINOR: checks: allow external checks in backend sections

MINOR: checks: allow external checks in backend sections

Previously, external checks required to find at least one listener in order to
pass the <proxy_address> and <proxy_port> arguments to the external script.
It prevented from declaring external checks in backend sections and haproxy
rejected the configuration.

The listener is now optional and values "NOT_USED" are passed if no listener is
found. For instance, this is the case with a backend section.

This is specific to the 1.6 branch.