Feature request: support for safe channels

There does not appear to be support for "safe" channels in master. These are channels with the prefix !, described in RFC 2811 section 3.2.

Adding ! to chantypes in irc_client.cpp makes it possible to join these, but when using the short (user-friendly) name, the resulting INVITE should be handled automatically for complete support. The underlying unique name should probably also be hidden (this is what irssi does).

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