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/winsup.h

@@ -134,7 +134,7 @@ extern int cygserver_running;
 #include "debug.h"
 
 /* Events/mutexes */
-extern HANDLE title_mutex;
+extern HANDLE tty_mutex;
 
 /**************************** Convenience ******************************/