MINOR: limits: explain a bit better what to do when fd limits are exceeded

MINOR: limits: explain a bit better what to do when fd limits are exceeded

As shown in github issue #3191, the error message shown when FD limits
are exceeded is not very useful as-is, since the current hard limit is
not displayed, and no suggestion is made about what to change in the
config. Let's explain about maxconn/ulimit-n/fd-hard-limit, suggest
dropping them or setting them to a context-based value at roughly 49%
of the current limit minus the known used FDs for listeners and checks.
This allows common "large" hard limits to report mostly round maxconns.
Example:

  [ALERT]    (25330) : [haproxy.main()] Cannot raise FD limit to 4001020,
  current limit is 1024 and hard limit is 4096. You may prefer to let
  HAProxy adjust the limit by itself; for this, please just drop any
  'maxconn' and 'ulimit-n' from the global section, and possibly add
  'fd-hard-limit' lower than this hard limit. You may also force a new
  'maxconn' value that is a bit lower than half of the hard limit minus
  listeners and checks. This results in roughly 1500 here.