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/multihandle.h

@@ -98,6 +98,7 @@ struct Curl_multi {
   struct uint_tbl xfers; /* transfers added to this multi */
   /* Each transfer's mid may be present in at most one of these */
   struct uint_bset process; /* transfer being processed */
+  struct uint_bset dirty; /* transfer to be run NOW, e.g. ASAP. */
   struct uint_bset pending; /* transfers in waiting (conn limit etc.) */
   struct uint_bset msgsent; /* transfers done with message for application */