libkyahoo: Apply some fixes related to sending and receiving files.

From the ReviewBoard request:

Receive part:
a) protocol (as reading from various places) suggests that first a
HEAD and then GET should be done. Current code was asking them in
parallel.
b) fixed the way sending the header was done.

Sending:
a) changed to an async method of doing the send
b) send buffer size is dynamic to try to send as much as possible
c) ... various - made it work.

Maybe fixes also #194833 although I'm not very sure what is that one
about.

Original patch by Cristi Posoiu <cristi.posoiu AT gmail>, thanks!

Cannot backport due to the string additions.

FIXED-IN: 4.7.0