libwebsockets

ss: multipart without processing

Change the default to not process multipart mime at SS layer.

If it's desired, then set "http_multipart_ss_in" true in the policy on the streamtype.

To test, use lws-minimal-secure-streams-avs, which uses SS processing as it is.

To check it without the processing, change #if 1 to #if 0 around the policy for
"http_multipart_ss_in" in both places in avs.c, and also enable the hexdump in ss_avs_metadata_rx()
also in avs.c, and observe the multipart framing is passed through unchanged.