multi: add dirty bitset

Add a bitset `dirty` to the multi handle. The presence of a transfer int
he "dirty" set means: this transfer has something to do ASAP.

"dirty" is set by multiplexing protocols like HTTP/2 and 3 when
encountering response data for another transfer than the current one.
"dirty" is set by protocols that want to be called.

Implementation:

* just an additional `uint_bset` in the multi handle
* `Curl_multi_mark_dirty()` to add a transfer to the dirty set.
* `multi_runsingle()` clears the dirty bit of the transfer at
   start. Without new dirty marks, this empties the set after
   al dirty transfers have been run.
* `multi_timeout()` immediately gives the current time and
   timeout_ms == 0 when dirty transfers are present.
* multi_event: marks all transfers tracked for a socket as dirty.
  Then marks all expired transfers as dirty. Then it runs
  all dirty transfers.

With this mechanism:

* Most uses of `EXPIRE_RUN_NOW` are replaced by `Curl_multi_mark_dirty()`
* `Curl_multi_mark_dirty()` is cheaper than querying if a transfer is
  already dirty or set for timeout. There is no need to check, just do it.
* `data->state.select_bits` is eliminated. We need no longer to
  simulate a poll event to make a transfer run.

Closes #17662

lib/imap.c

@@ -1347,9 +1347,6 @@ static CURLcode imap_state_fetch_resp(struct Curl_easy *data,
     else {
       /* IMAP download */
       data->req.maxdownload = size;
-      /* force a recv/send check of this connection, as the data might've been
-       read off the socket already */
-      data->state.select_bits = CURL_CSELECT_IN;
       Curl_xfer_setup1(data, CURL_XFER_RECV, size, FALSE);
     }
   }