Print the reason for the exit.

demo/stderr.py

@@ -22,10 +22,13 @@ while x.loop():
     elif x.timeout():
         pass
     elif x.eof(user):
+        print "EOF on user"
         break
     elif x.eof(sh_err):
+        print "EOF on stderr"
         break
     elif x.eof(sh):
+        print "EOF on stdout"
         break
 
 sh.close()