Make FTP transfer buffer size configurable

the current value is fixed value 16KB in
source/filezilla/TransferSocket.cpp
It's too small for large file transfer in fast LAN/WAN network.
It would be good we could config the value up to 4MB.

Thanks for your suggestion. I'll look into it.

I have done some tests and I do not see any speed difference even with 1MB buffer.
I can send you a custom build if your want to test in your environment. Contact me at my email address.

Note that the OS-level buffer is already set to 4MB. That's probably why the application buffer size has little if any impact.