Ensuring that an application is fork safe can be complex, and
in case of mistake, one of the error condition is shared file
descriptors, causing one worker to read packets from another
which could have dire consequences.

Rather than to track down all the file descriptors that need
reopening, `close_all_fds!` allows to automatically close all
file descriptors that aren't explicitly marked as needing to
be kept.

And in case of mistake, the error condition is that trying to
read or write into that file descriptor will fail, which is
much less critical than to read packets from another worker.