pie/piegen: make -f explicit parameter

One may specify other parameters, but not 'f'.
Piegen should output usage() instead of trying to open "file.o"
as -f is explicit.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>