[console] Allow '?' as an intermediate byte in ANSI escape sequences

The ANSI escape sequences to show and hide the cursor take the form
"<ESC>[?25h" and "<ESC>[?25l" respectively.  iPXE currently treats the
'?' character as the final byte.  Fix by explicitly treating '?' as an
intermediate byte.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[mucurses] Implement curs_set() to control cursor visibility

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[settings] Hide cursor when not actively editing a setting

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[menu] Hide cursor when displaying menu

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[fbcon] Add support for displaying a cursor

Signed-off-by: Michael Brown <mcb30@ipxe.org>