Spelling fix: usefull --> useful.

Thank you Curtis Brown for the patch.

This resolves bug #27052.

src/NEWS.3.5

@@ -77,7 +77,7 @@
   document.
 
 * Screen can now be started detached (screen -d -m -S sockname).
-  This is usefull if you want to start screen in your /etc/rc file
+  This is useful if you want to start screen in your /etc/rc file
   (e.g. as a console multiplexer)
 
 * Console grabbing added ('console on' command). 

src/mark.c

@@ -771,7 +771,7 @@ processchar:
             LGotoPos(flayer, cx, W2D(cy));
 	  break;
 	case '@':
-	  /* it may be usefull to have a key that does nothing */
+	  /* it may be useful to have a key that does nothing */
 	  break;
 	case '%':
 	  rep_cnt--;

src/window.c

@@ -642,7 +642,7 @@ struct NewWindow *newwin;
 #ifdef MULTIUSER
   /*
    * This is dangerous: without a display we use creators umask
-   * This is intended to be usefull for detached startup.
+   * This is intended to be useful for detached startup.
    * But is still better than default bits with a NULL user.
    */
   if (NewWindowAcl(p, display ? D_user : users))