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#12898
Stefan Eissing