MINOR: init: move the maxsock calculation code to compute_ideal_maxsock()

MINOR: init: move the maxsock calculation code to compute_ideal_maxsock()

The maxsock value is currently derived from global.maxconn and a few other
settings, some of which also depend on global.maxconn. This makes it
difficult to check if a limit is already too high or not during the maxconn
automatic sizing.

Let's move this code into a new function, compute_ideal_maxsock() which now
takes a maxconn in argument. It performs the same operations and returns
the maxsock value if global.maxconn were to be set to that value. It now
replaces the previous code to compute maxsock.