[dcc2] SIDs and connections to self.

[myndzi]

I ran into this annoying little ambiguity while working on
implementing DCC2 stuff.

It makes sense to keep a single consolidated list of nick,sid pairs so
that you do not end up with two different connections (one initiated
by you, and one by them) with the same SID. However when you do this,
sending DCC2 requests to yourself becomes troublesome -- obviously the
nick and SID will be the same.

Since you can't really tell if a DCC2 Accept CTCP is for a transaction
*you* initiated or one *they* initiated, there being two possibilities
here (direct reply to a DCC2 Publish, or a reply to a DCC Accept), it
seems that some thought should be put into this. This is another good
reason to change the name of each CTCP or add some sort of state
counter.

-myndzi