Move verify_vc_kbmode() to terminal-util.c as vt_verify_kbmode()

src/basic/terminal-util.h

@@ -155,6 +155,7 @@ int openpt_in_namespace(pid_t pid, int flags);
 int open_terminal_in_namespace(pid_t pid, const char *name, int mode);
 
 int vt_default_utf8(void);
+int vt_verify_kbmode(int fd);
 int vt_reset_keyboard(int fd);
 int vt_restore(int fd);
 int vt_release(int fd, bool restore_vt);