Implement SVSNICK command to change remote nicknames

Implement SVSNICK command to change remote nicknames

The SVSNICK command allows other servers (and services on
"pseudo-servers") to forcefully change nicknames of remote users.

Syntax: ":<from> SVSNICK <oldnick> <newnick>"

The SVSNICK command itself doesn't change the nickname, but it becomes
forwarded to the server to which the user is connected to. And then this
server initiates the real nickname changing using regular NICK commands.

This allows to still run networks with old servers not supporting the
SVSNICK command, because SVSNICK commands for nicknames on such servers
are silently ignored and don't cause a desync of the network.