Send NICK commands with prefix of (target) user

Send NICK commands with prefix of (target) user

Now NICK commands are always generated using the prefix of the target
user, even when the nickname change has been initiated by some other
(pseudo) server or using the SVSNICK command. In this case, the prefix
of the initiator has been used, but this isn't compatible with clients
(at least weechat and irssi don't handle such NICK commands correctly).