MINOR: init: make the maxpipe computation more accurate

MINOR: init: make the maxpipe computation more accurate

The default number of pipes is adjusted based on the sum of frontends
and backends maxconn/fullconn settings. Now that it is possible to have
a null maxconn on a frontend to indicate "unlimited" with commit
c8d5b95e6 ("MEDIUM: config: don't enforce a low frontend maxconn value
anymore"), the sum of maxconn may remain low and limited to the only
frontends/backends where this limit is set.

This patch considers this new unlimited case when doing the check, and
automatically switches to the default value which is maxconn/4 in this
case. All the calculation was moved to a distinct function for ease of
use. This function also supports returning unlimited (-1) when the
value depends on global.maxconn and this latter is not yet set.