posix.cfg: Added support for umask().

cfg/posix.cfg

@@ -1000,6 +1000,15 @@ The function 'mktemp' is considered to be dangerous due to race conditions and s
       <not-bool/>
     </arg>
   </function>
+  <!-- mode_t umask(mode_t mask); -->
+  <function name="umask">
+    <noreturn>false</noreturn>
+    <leak-ignore/>
+    <returnValue type="mode_t"/>
+    <arg nr="1" direction="in">
+      <not-uninit/>
+    </arg>
+  </function>
   <!-- int fchmod(int fildes, mode_t mode); -->
   <function name="fchmod">
     <noreturn>false</noreturn>