downstream: disallow '.' in nicknames

See [1] and [2].

[1]: https://github.com/ircdocs/modern-irc/pull/148
[2]: eff6dd242b/irc/strings.go (L29)

downstream.go

@@ -215,7 +215,8 @@ func updateNetworkAttrs(record *Network, attrs irc.Tags, subcommand string) erro
 //     '?' breaks masks
 //     '$' breaks server masks in PRIVMSG/NOTICE
 //     ',' breaks lists
-const illegalNickChars = " :@!*?$,"
+//     '.' is reserved for server names
+const illegalNickChars = " :@!*?$,."
 
 // permanentDownstreamCaps is the list of always-supported downstream
 // capabilities.