multi*: Could handle properly ES KEYFRAME combined with headers

Submitted by Nicola @drakkan

Link to original bug (#749649)

Description

Please take a look at the file here:

http://195.250.34.59/temp/734124.gdp

and use a pipeline like this:

filesrc ! gdpdepay ! h264parse ! video/x-h264,stream-format=avc,alignment=au ! fakesink silent=false

you will see that all keyframes have header flag too

fakesink0:sink) (338607 bytes, dts: 0:00:15.118625403, pts: 0:00:15.152625403, duration: 0:00:00.033333333, offset: 7302216, offset_end: -1, flags: 00000400 header ) 0x7f47f008cb20

in h264parse, here

http://cgit.freedesktop.org/gstreamer/gst-plugins-bad/tree/gst/videoparsers/gsth264parse.c#n1878

both h264parse->keyframe and h264parse->header are true, since there is no explicit flag for a keyframe how is supposed an app to find them? I based my code on the one from multihandlesink

http://cgit.freedesktop.org/gstreamer/gst-plugins-base/tree/gst/tcp/gstmultihandlesink.c#n1140

that will not recognize keyframes in this case