mpegvideo_enc: drop outdated copy_picture_attributes() in favour of a modern av_frame_copy_props()

Signed-off-by: Anton Khirnov <anton@khirnov.net>
libavutil: add avpriv_open() to open files with close-on-exec flag

Signed-off-by: Anton Khirnov <anton@khirnov.net>
libavutil: use avpriv_open()

Signed-off-by: Anton Khirnov <anton@khirnov.net>
libavcodec: use avpriv_open()

Signed-off-by: Anton Khirnov <anton@khirnov.net>
libavdevice: use avpriv_open()

Signed-off-by: Anton Khirnov <anton@khirnov.net>
libavformat: use avpriv_open()

Signed-off-by: Anton Khirnov <anton@khirnov.net>