posix.cfg: Improved configuration fo fileno().

cfg/posix.cfg

@@ -3710,13 +3710,14 @@ The function 'mktemp' is considered to be dangerous due to race conditions and s
       <not-bool/>
     </arg>
   </function>
+  <!-- http://man7.org/linux/man-pages/man3/fileno.3p.html -->
   <!-- int fileno(FILE *stream); -->
   <function name="fileno">
     <returnValue type="int"/>
     <use-retval/>
     <noreturn>false</noreturn>
     <leak-ignore/>
-    <arg nr="1">
+    <arg nr="1" direction="inout">
       <not-null/>
       <not-uninit/>
       <not-bool/>