BUG/MINOR: init: make the automatic maxconn consider the max of soft/hard limits

James Stroehmann reported something working as documented but that can
be considered as a regression in the way the automatic maxconn is
calculated from the process' limits :

  https://www.mail-archive.com/haproxy@formilux.org/msg36523.html

The purpose of the changes in 2.0 was to have maxconn default to the
highest possible value permitted to the user based on the ulimit -n
setting, however the calculation starts from the soft limit, which
can be lower than what users were allowed to with previous versions
where the default value of 2000 would force a higher ulimit -n as
long as it fitted in the hard limit.

Usually this is not noticeable if the user changes the limits, because
quite commonly setting a new value restricts both the soft and hard
values.

Let's instead always use the max between the hard and soft limits, as
we know these values are permitted. This was tried on the following
setup:

  $ cat ulimit-n.cfg
  global
    stats socket /tmp/sock1 level admin
  $ ulimit -n
  1024

Before the change the limits would show like this:

  $ socat - /tmp/sock1 <<< "show info" | grep -im2 ^Max
  Maxsock: 1023
  Maxconn: 489

After the change the limits are now much better and more in line with
the default settings in earlier versions:

  $ socat - /tmp/sock1 <<< "show info" | grep -im2 ^Max
  Maxsock: 4095
  Maxconn: 2025

The difference becomes even more obvious when running moderately large
configs with hundreds of checked servers and hundreds of listeners:

  $ cat ulimit-n.cfg
  global
    stats socket /tmp/sock1 level admin

  listen l
    bind :10000-10300
    server-template srv- 300 0.0.0.0 check disabled

          Before   After
  Maxsock  1024    4096
  Maxconn  189     1725

This issue is tagged as minor since a trivial config change fixes it,
but it would help new users to have it backported as far as 2.0.

src/haproxy.c

@@ -1417,7 +1417,7 @@ static int compute_ideal_maxconn()
 	int ssl_sides = !!global.ssl_used_frontend + !!global.ssl_used_backend;
 	int engine_fds = global.ssl_used_async_engines * ssl_sides;
 	int pipes = compute_ideal_maxpipes();
-	int remain = rlim_fd_cur_at_boot;
+	int remain = MAX(rlim_fd_cur_at_boot, rlim_fd_max_at_boot);
 	int maxconn;
 
 	/* we have to take into account these elements :