Fixing File Transfer in 3.0

[Michael McConville]

File transfer has been broken in 3.0 for a while:

	https://developer.pidgin.im/ticket/16001

I found two issues:

	* the recipient was accounting for each received byte twice (the
	  obvious and reported issue)
	* redundant calls to the transfer completion function were
	  slightly, non-deterministically truncating files

This fixes both for me on XMPP:

	https://hg.pidgin.im/soc/2015/mmcc/main/rev/acd5549cf5d6

Because each protocol uses the file transfer API in its own way, this
might break some. However, I'd call this "unambiguously less broken".
Any objections?