Fixed copyrights in sources

Was tired of seeing corporate advertising in the source files; this goes against what we agreed when we switched to shared copyright, was making a mess of the sources, and was unfair to the bulk of individual authors. I removed it all, including iMatix's original copyright statements. Corporate advertising can go into AUTHORS.

OK, this PR has gotten larger. I'm experimentally adding encryption/authentication to the library, and am doing small cleanups as I see fixes to make. Protocol revision is bumped; I'll roll that back if we decide to not implement security.