nexedi/erp5@39369169 added the functionality to ERP5 to
set its soft limit of allowed open file descriptors to the system wide hard limit.
This parameter is useful for Wendelin based instances where the 1024
limit is easily reached.

With this patch, this parameter can also be set via SlapOS, which
simplifies usage of the Wendelin SR.

/reviewed-by @jerome, @jm, @rafael, @vpelletier
/reviewed-on nexedi/slapos!1465

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This patch was already added in nexedi/slapos!1451, but
removed (by force push), due to issues discussed in nexedi/slapos!1451 (comment 193296).