lib: introduce ui_notify_user()

ui_notify_user(frmt, ...) prints formatted notification string at
(x:1, y:0) WINDOW position.

use fixed UI_NOTIFY_BUFF_SZ size buffer, because there seems to be
no ncurses() *print function which takes int x, int y WINDOW
coordinates and va_list.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>