Fix: use 'open files' soft limit to preallocate arena

The hard limit can sometimes be be very high (e.g. 1 billion) and trying
to preallocate that much virtual memory is failing. Let's use the soft
limit for starters, then we can revisit the problem by allocating the
arena into blocks instead (no upper limit at the expense of a bounds
check).

src/crystal/system/unix/evented/event_loop.cr

@@ -39,7 +39,7 @@ module Crystal::Evented
     if LibC.getrlimit(LibC::RLIMIT_NOFILE, out rlimit) == -1
       raise RuntimeError.from_errno("getrlimit(RLIMIT_NOFILE)")
     end
-    rlimit.rlim_max.clamp(..Int32::MAX).to_i32!
+    rlimit.rlim_cur.clamp(..Int32::MAX).to_i32!
   end
 end