This is done by making sure each individual transfer is first added to a
linked list as then they can be performed serially, or at will, in
parallel.
Closes#3804
Now FILE transfers send headers to the header callback like HTTP and
other protocols. Also made curl_easy_getinfo(...CURLINFO_PROTOCOL...)
work for FILE in the callbacks.
Makes "curl -i file://.." and "curl -I file://.." work like before
again. Applied the bold header logic to them too.
Regression from c1c2762 (7.61.0)
Reported-by: Shaun Jackman
Fixes#3083Closes#3101
