Add Lua REPL mode and multiline edit function in the mode.

Typing "@repl" after Ctrl-t r, enter Lua REPL mode.
Typing "@exit" in REPL mode will return you to normal mode.

Note:
- the determination of continuation lines is not done automatically, and
  if the end of a line is \, it is determined to be a continuation
  instruction.
- In REPL mode, tio's main loop is blocked. (Ctrl-t q works.)

Example:
>> t = {1,2,3,4,5}
>> for _,v in ipairs(t) do\
>>     print(v,"\r")\
>> end

Implementation improvements:
- Add tty_init() and script_interp_init() in order to only once
  initialization.
- Fix script function to work without device_fd.

src/tty.h

@@ -76,6 +76,7 @@ void stdout_configure(void);
 void stdin_configure(void);
 void tty_configure(void);
 void tty_reconfigure(void);
+void tty_init(void);
 int tty_connect(void);
 void tty_wait_for_device(void);
 void list_serial_devices(void);