[BUG] global.tune.maxaccept must be limited even in mono-process mode

[BUG] global.tune.maxaccept must be limited even in mono-process mode

On overloaded systems, it sometimes happens that hundreds or thousands
of incoming connections are queued in the system's backlog, and all get
dequeued at once. The problem is that when haproxy processes them and
does not apply any limit, this can take some time and the internal date
does not progress, resulting in wrong timer measures for all sessions.

The most common effect of this is that all of these sessions report a
large request time (around several hundreds of ms) which is in fact
caused by the time spent accepting other connections. This might happen
on shared systems when the machine swaps.

For this reason, we finally apply a reasonable limit even in mono-process
mode. Accepting 100 connections at once is fast enough for extreme cases
and will not cause that much of a trouble when the system is saturated.
(cherry picked from commit f49d1df25cf794b8801d919fda20266d90981c78)
(cherry picked from commit d80d63b6df4fe2d8711a83f88c51a7e3a0e139ed)