libeio bugfix: want_poll should be called if breaking on maxreq

Reported by shansen and hassox
http://github.com/ry/node/issues#issue/38

Will send upstream.

deps/libeio/eio.c

@@ -548,6 +548,9 @@ static int etp_poll (void)
         }
     }
 
+  if (want_poll_cb)
+    want_poll_cb ();
+
   errno = EAGAIN;
   return -1;
 }

test/mjsunit/test-many-parallel-eio-jobs.js

@@ -0,0 +1,20 @@
+process.mixin(require("./common"));
+var testTxt = path.join(fixturesDir, "x.txt");
+var posix = require('posix');
+
+setTimeout(function () {
+  // put this in a timeout, just so it doesn't get bunched up with the
+  // require() calls..
+  N = 30;
+  for (var i=0; i < N; i++) {
+    puts("start " + i);
+    posix.cat(testTxt).addCallback(function(data) {
+      puts("finish");
+    }).addErrback(function (e) {
+      puts("error! " + e);
+      process.exit(1);
+    });
+  }
+}, 100);
+
+