Warn if you're being a public relay but have too-low file descriptor limit
Inspired by discussion on #3030 (moved): if we're a public relay, we should check if ulimit -n is too low (under 8192 I'm thinking, based on debian's init script), and warn (and recommend using a package) if so.
This is to help people who run Tor from tarball and don't realize all the fixes you have to do manually to do it right.
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