libknc: make knc_get_ostream_contig() able to return short buffers.
libknc: use knc_get_ostream_contig() in knc_process_out().

After the last commit, this is now possible to do.  This enables gss packets
to cross allocation boundaries without creating new packets which should
ensure that we end up with packets that are closer to ctx->gssmaxpacket.
libknc: reduce default read size to 16KB.
libknc: simplify read_packet() a bit.