Add an alert if the system is close to running out of file descriptors

As was found in MB-27594, running out of file descriptors can cause epmd to crash which means that if file descriptors are freed up and the system recovers, ns_server isn't able to get back to normal (because of the missing epmd).

Obviously, in this case the right thing to do is to restart the node - but it may not be entirely clear to do this. It seems like a good idea to warn users ahead of time that they are close to running out of FDs so they could head the problem off and in the event that it happens anyway, seeing the alerts in the log should speed up the diagnosis and the remedial actions to be taken.