Implement zero-copy and make the buffer size configurable.

Implement zero-copy and make the buffer size configurable.

Original commit message from CVS:
Patch by: Wouter Cloetens <wouter at mind dot be>
* configure.ac:
* ext/soup/gstsouphttpsrc.c: (gst_soup_http_src_cancel_message),
(gst_soup_http_src_finished_cb), (gst_soup_http_src_chunk_free),
(gst_soup_http_src_chunk_allocator),
(gst_soup_http_src_got_chunk_cb), (gst_soup_http_src_create),
(gst_soup_http_src_start), (gst_soup_http_src_set_proxy):
* ext/soup/gstsouphttpsrc.h:
Implement zero-copy and make the buffer size configurable.
Prefix proxy URIs with "http://" if they don't start with it
already and catch errors earlier, fixes hanging in some situations.
Fixes bug #514948.