Add support for overridable VM name

- A global variable added as vm_name
- This variable can be filled with the `--vmname "Name"` or `-V "Name"` parameter.
- If there are no such a parameter definied this variable will filled up with the directory name.
- The Discord module displays this global variable, as VM name.
- Various 86Box managers can use this feature to display fancy VM names, instead of GUID folder names.
- This variable can be easily used later for adding cool things, like the VM name in title bar, etc.

src/include/86box/86box.h

@@ -73,6 +73,7 @@ extern uint64_t	unique_id;
 extern uint64_t	source_hwnd;
 #endif
 extern char	log_path[1024];			/* (O) full path of logfile */
+extern char	vm_name[1024];			/* (O) display name of the VM */
 
 
 extern int	window_w, window_h,		/* (C) window size and */