plugins/elements/gstfilesink.*: Check if we can seek in the file instead of assuming...

plugins/elements/gstfilesink.*: Check if we can seek in the file instead of assuming we always can. Post an error whe...

Original commit message from CVS:
* plugins/elements/gstfilesink.c: (gst_file_sink_open_file),
(gst_file_sink_close_file), (gst_file_sink_do_seek),
(gst_file_sink_event), (gst_file_sink_render):
* plugins/elements/gstfilesink.h:
Check if we can seek in the file instead of assuming
we always can. Post an error when we are asked to seek in a
non-seekable file (like a fifo). Fixes #343312.
Some cleanups.