non 420P grab fix - suppressed unused 'sys/poll.h' header - do not put error...

non 420P grab fix - suppressed unused 'sys/poll.h' header - do not put error if audio or video stream not found (no longer needs to use -an or -vn when it is obvious) - display error message when image filename does not contain '%d'


Originally committed as revision 123 to svn://svn.ffmpeg.org/ffmpeg/trunk