Added *promise-keep-specials*

Sometimes it's desirable to keep some kind of context available via special vars during execution of a promise chain. For instance one may want to keep http request related context while serving a request in a web app. I added *promise-keep-specials* special var that can be used for such purposes:
```cl
(pushnew '*my-var* bb:*promise-keep-specials*)
```

Several notes:
1. Due to extra wrapping of callbacks/errbacks, this slightly changes the semantics of attach-errback - namely, it no longer can detect whether the callback was already added. Don't know whether that's critical. For purposes of promise forwarding, the non-wrapping version named do-attach-errback is used.
2. I thought about making alet/alet* bindings of special vars work in such way that these vars are auto-registered in *promise-keep-specials* while executing any code related to that alet/alet* form, but couldn't quite wrap my head around it (how to do it properly and whether this is right thing at all)
3. (As a side note) there was an unused gensym var in alet macro, fixed it too.