ui/curses: Fix pageup/pagedown on -curses

Current KEY_NPAGE/KEY_PPAGE handling is broken on -curses. Those uses
"GREY", but "KEY_MASK" masked out "GREY".

To fix, we have to use correct mask value - SCANCODE_KEYMASK.

Then, this adds support of "shift + pageup/pagedown". With this,
-curses mode can use scroll-up/down as usual like other display modes.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>