ISUPPORT to indicate which commands accept masks

[emersion]

Some commands (like WHO) sometimes accept masks, but without any way for the client to discover this functionality.

Add an informational ISUPPORT (ala TARGMAX) to indicate which commands support masks.

[progval]

note that we'll need some subtlety here, because some commands accept multiple types of targets (nick/channel/server), and may only allow masks for some.

There's also the issue of WHO masks being unclear on what is being matches (eg. some servers include GECOS)

[emersion]

List of commands which might accept masks for nicks and channels: NAMES, LIST (already covered by ELIST), WHO, WHOIS, WHOWAS, USERHOST.

[progval]

WHOWAS too