Rename getch2....c/h to terminal....c/h

"getch2" really tells nothing about what the heck this code does. It'd
be even worse when moving the rest of terminal handling code there.

osdep/terminal-unix.c

@@ -45,7 +45,7 @@
 #include "bstr/bstr.h"
 #include "input/input.h"
 #include "input/keycodes.h"
-#include "getch2.h"
+#include "terminal.h"
 
 #if HAVE_TERMIOS
 static volatile struct termios tio_orig;

osdep/terminal-win.c

@@ -30,7 +30,7 @@
 #include <windows.h>
 #include "input/keycodes.h"
 #include "input/input.h"
-#include "getch2.h"
+#include "terminal.h"
 
 int mp_input_slave_cmd_func(int fd, char *dest, int size)
 {