private-libwebsockets.h: don't need body_index!
Just count content length remaining to copy:

 - Fulfills body_index and content_length_seen rolls
 - Allows us to read only the appropriate amount from the service_buffer in cases where len > content_remain
server.c: No body_index anymore.
handshake.c:

WSI_STATE_HTTP_BODY:
 * **use memcpy to copy body data**
 * stash post buffer size
 * track content_length remain
 * don't invoke LWS_CALLBACK_HTTP_BODY_COMPLETION without callback
 * moved "return 0;" for websockets/breaks above http_complete label
 * removed body spill <-- this might be the next request from a keep-alive connection