Clarify licensing

[mdempsky]

In the top-level directory there's a LICENSE file that suggests the code is GPLv3, but then the "receiver" and "sender" subdirectories have their own LICENSE.txt files with a copy of the 3-clause BSD license. The inherited UDT code all reiterates the 3-clause licensing, but the newly added code is ambiguous whether you intend for it to be GPLv3 or BSD. (I'd encourage you to use BSD for consistency with UDT, but that's ultimately your decision.)

Could you please add copyright and licensing notices to the remaining files? E.g., sender/app/cc.h, sender/app/incast_client.cpp, and so on.