less: switch off nonblock on kbd_fd before exit

less: switch off nonblock on kbd_fd before exit

This is only necessary if we use stdout fd.

function                                             old     new   delta
less_exit                                             32      51     +19
less_main                                           2540    2543      +3
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(add/remove: 0/0 grow/shrink: 2/0 up/down: 22/0)               Total: 22 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>