MINOR: listener: add nbconn kw for reverse connect

MINOR: listener: add nbconn kw for reverse connect

Previously, maxconn keyword was reused for a specific usage on reverse
HTTP binds to specify the number of active connect to proceed. To avoid
confusion, introduce a new dedicated keyword 'nbconn' which is specific
to reverse HTTP bind.

This new keyword is forbidden for non-reverse listener. A fatal error is
emitted during config parsing if this rule is not respected. It's safe
because it's also forbidden to mix standard and reverse addresses on the
same bind line.

Internally, nbconn value will be reassigned to 'maxconn' member of
bind_conf structure. This ensures that listener layer will automatically
reenable the preconnect task each time a connection is closed.