lib: enhance client reader resume + rewind

- update client reader documentation
- client reader, add rewind capabilities
    - tell creader to rewind on next start
    - Curl_client_reset() will keep reader for future rewind if requested
    - add Curl_client_cleanup() for freeing all resources independent of
      rewinds
    - add Curl_client_start() to trigger rewinds
    - move rewind code from multi.c to sendf.c and make part of
      "cr-in"'s implementation
- http, move the "resume_from" handling into the client readers
    - the setup of a HTTP request is reshuffled to follow:
      * determine method, target, auth negotiation
      * install the client reader(s) for the request, including crlf
        conversions and "chunked" encoding
      * apply ranges to client reader
      * concat request headers, upgrades, cookies, etc.
      * complete request by determining Content-Length of installed
        readers in combination with method
      * send
    - add methods for client readers to
      * return the overall length they will generate (or -1 when unknown)
      * return the amount of data on the CLIENT level, so that
        expect-100 can decide if it want to apply itself
      * set a "resume_from" offset or fail if unsupported
    - struct HTTP has become largely empty now
- rename `Client_reader_*` to `Curl_creader_*`

Closes #13026

scripts/singleuse.pl

@@ -46,7 +46,8 @@ my %wl = (
     'Curl_xfer_write_resp' => 'internal api',
     'Curl_creader_def_init' => 'internal api',
     'Curl_creader_def_close' => 'internal api',
-    );
+    'Curl_creader_def_total_length' => 'internal api',
+);
 
 my %api = (
     'curl_easy_cleanup' => 'API',