(libT) move the tr_peer declaration from peer-mgr.h to peer-common.h

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13947 f4695dd4-2c0a-0410-b89c-da849a56a58e
move tr_peer.encryptionPreference to tr_peermsgs.encryptionPreference

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13948 f4695dd4-2c0a-0410-b89c-da849a56a58e