BUG/MINOR: fix maxaccept computation according to the frontend process range

BUG/MINOR: fix maxaccept computation according to the frontend process range

commit 7c0ffd23 is only considering the explicit use of the "process" keyword
on the listeners. But at this step, if it's not defined in the configuration,
the listener bind_proc mask is set to 0. As a result, the code will compute
the maxaccept value based on only 1 process, which is not always true.

For example :
  global
    nbproc 4

  frontend test
    bind-process 1-2
    bind :80

Here, the maxaccept value for the "test" frontend was set to the global
tune.maxaccept value (default to 64), whereas it should consider 2 processes
will accept connections. As of the documentation, the value should be divided
by twice the number of processes the listener is bound to.

To fix this, we can consider that if no mask is set to the listener, we take
the frontend mask.

This is not critical but it can introduce unfairness distribution of the
incoming connections across the processes.

It should be backported to the same branches as commit 7c0ffd23 (1.6 and 1.5
were in the scope).