ERP5/zopewsgi: Add option to set soft limit of open file descriptors to hard limit

The default soft limit of open file descriptors is usually set to 1024
in order to avoid breaking old software which still uses select. In many
projects we may need a higher limit: particularly in Wendelin based
projects we easily reach this limit. Before this patch it was therefore
necessary to either patch ERP5 in the project specific SR or to manually
increase the limit of the zope processes (or the parent supervisor) with
a tool like prlimit [1]. With this patch it becomes possible to increase the
soft limit to the hard limit with a command line argument of the zopewsgi bin.
This simplifies setting the soft limit for any Wendelin project.

[1] https://man7.org/linux/man-pages/man1/prlimit.1.html

/reviewed-by @vpelletier, @jerome
/reviewed-on nexedi/erp5!1827