gst/: Set pads to FLUSHING when they are created. Check, warn and fix when a demuxer adds an inactive pad to itself w...

Original commit message from CVS:
* gst/gstelement.c: (gst_element_add_pad):
* gst/gstghostpad.c: (gst_ghost_pad_new_full):
* gst/gstpad.c: (gst_pad_init):
Set pads to FLUSHING when they are created. Check, warn and fix when a
demuxer adds an inactive pad to itself when running. Fixes #339326.