Change the name "title_mutex" to "tty_mutex" throughout.

* tty.h (tty_list::allocate_tty): Turn argument into a boolean.
* tty.cc (tty_list::init): Protect entire allocation operation with tty_mutex
(formerly title_mutex) to protect against allocation races.

winsup/cygwin/tty.h

@@ -125,7 +125,7 @@ class tty_list
 
 public:
   tty * operator [](int n) {return ttys + n;}
-  int allocate_tty (int n); /* n non zero if allocate a tty, pty otherwise */
+  int allocate_tty (bool); /* true if allocate a tty, pty otherwise */
   int connect_tty (int);
   void terminate ();
   void init ();