HTTP Version, Keep-alive support, No-copy POST

## Details:
This is a squashed commit from https://github.com/andrew-canaday/libwebsockets,
*dev/http_keepalive* branch (strategies changed a few times, so the commit
history is clutteread). This branch is submitted for clarity, but the other
can be used as a reference or alternative. A summary follows.

### private-libwebsockets:
 * added **enum http_version** to track HTTP/1.0 vs HTTP/1.1 requests
 * added **enum http_connection_type** to track keep-alive vs close
 * replaced *content_length_seen* and *body_index* with **content_remain**
 * removed **post_buffer** (see handshake.c modifications)

### libwebsockets.c: libwebsocket_close_and_free_session
 * removed post_buffer free

### parsers.c: libwebsocket_parse
 * switch state to WSI_TOKEN_SKIPPING after URI is complete to store version
 * delete *spill* label (unused)

### server.c: lws_handshake_server
 * add vars to track HTTP version and connection type
 * HTTP version defaults to 1.0
 * connection type defaults to 'close' for 1.0, keep-alive for 1.1
 * additional checks in **cleanup:** label:
   * if HTTP version string is present and valid, set enum val appropriately
   * override connection default with the "Connection:" header, if present
 * set state to WSI_STATE_HTTP_BODY if content_length > 0
 * return 0 on HTTP requests, unless LWS_CALLBACK_HTTP indicates otherwise

### handshake.c: 
 * add vars to track remaining content_length and body chunk size
 * re-arrange switch case order to facilitate creation of jump-table
 * added new labels:
   * **read_ok**: normal location reach on break from switch; just return 0
   * **http_complete**: check for keep-alive + init state, mode, hdr table
   * **http_new**: jump location for keep-alive when http_complete sees len>0
 * after libwebsocket_parse, jump to one of those labels based on state
 * POST body handling:
   * don't bother iterating over input byte-by-byte or using memcpy
   * just pass the relevant portion of the context->service_buffer to callback