feat(script): Warn if `ulimit -n` is too low
feat(script): Warn if `ulimit -n` is too low

A low `ulimit -n` (max number of open files) can result in very annoying
server errors. When they happen, it's too late: the only option is to
change `ulimit -n` and restart a new server.

To avoid that, let's show a warning if the value is too low.

adrienverge/coucharchive