1. add the option the code to be used under GPLv2 or GPLv3; previously only GPLv2 was allowed
2. add the "proxy option" as described in GPLv3 so we can add future licenses without having to bulk-edit everything again :)
3. remove the awkward "exception for MIT code in Mac client" clause; it was unnecessary and confusing.
This commit also changes tr_recentHistory from being a general-purpose tool to being a little more hardcoded for the only purpose it's used, in tr_peerMgr. If its files (history.[ch]) don't find any other "customers" in libtransmission, eventually it should be demoted to being a private helper class inside of peer-mgr.c and have the history.[ch] files removed from the build.
The Berne Convention says that the copyright year is moot, so instead of adding another year to each file as in previous years, I've removed the year altogether from the source code comments in libtransmission, gtk, qt, utils, daemon, and cli.
Juliusz's copyright notice in tr-dht and Johannes' copyright notice in tr-lpd have been left alone; it didn't seem appropriate to modify them.
