lib: client writer, part 2, accounting + logging

This PR has these changes:

Renaming of unencode_* to cwriter, e.g. client writers
- documentation of sendf.h functions
- move max decode stack checks back to content_encoding.c
- define writer phase which was used as order before
- introduce phases for monitoring inbetween decode phases
- offering default implementations for init/write/close

Add type paramter to client writer's do_write()
- always pass all writes through the writer stack
- writers who only care about BODY data will pass other writes unchanged

add RAW and PROTOCOL client writers
- RAW used for Curl_debug() logging of CURLINFO_DATA_IN
- PROTOCOL used for updates to data->req.bytecount, max_filesize checks and
  Curl_pgrsSetDownloadCounter()
- remove all updates of data->req.bytecount and calls to
  Curl_pgrsSetDownloadCounter() and Curl_debug() from other code
- adjust test457 expected output to no longer see the excess write

Closes #12184

lib/content_encoding.h

@@ -25,15 +25,10 @@
  ***************************************************************************/
 #include "curl_setup.h"
 
-struct contenc_writer;
+struct Curl_cwriter;
 
 char *Curl_all_content_encodings(void);
 
 CURLcode Curl_build_unencoding_stack(struct Curl_easy *data,
                                      const char *enclist, int is_transfer);
-CURLcode Curl_unencode_write(struct Curl_easy *data,
-                             struct contenc_writer *writer,
-                             const char *buf, size_t nbytes);
-void Curl_unencode_cleanup(struct Curl_easy *data);
-
 #endif /* HEADER_CURL_CONTENT_ENCODING_H */