lib: move client writer into own source

Refactoring of the client writer that passes the data to the
client/application's callback functions.

- split out into own source cw-out.[ch] from sendf.c

- move tempwrite and tempcount from data->state into the context of the
  client writer

- redesign the 3 tempwrite dynbufs as a linked list of dynbufs. On
  paused transfers, this allows to "record" interleaved HEADER/BODY
  chunks to be "played back" in the same order on unpausing.

- keep the overall size limit of all buffered data to DYN_PAUSE_BUFFER.
  On exceeding that, return CURLE_TOO_LARGE instead of
  CURLE_OUT_OF_MEMORY as before.

- add method to be called when a transfer is DONE to allow writing of
  any data still buffered

- when paused, record HEADER writes exactly as they come for later
  playback. HEADERs are documented to be written one-by-one.

Closes curl#12898

lib/Makefile.inc

@@ -138,6 +138,7 @@ LIB_CFILES =         \
   curl_sspi.c        \
   curl_threads.c     \
   curl_trc.c         \
+  cw-out.c           \
   dict.c             \
   doh.c              \
   dynbuf.c           \
@@ -283,6 +284,7 @@ LIB_HFILES =         \
   curl_threads.h     \
   curl_trc.h         \
   curlx.h            \
+  cw-out.h           \
   dict.h             \
   doh.h              \
   dynbuf.h           \