h2/h3: replace `state.drain` counter with `state.dselect_bits`

h2/h3: replace `state.drain` counter with `state.dselect_bits`

- `drain` was used by http/2 and http/3 implementations to indicate
  that the transfer requires send/recv independant from its socket
  poll state. Intended as a counter, it was used as bool flag only.
- a similar mechanism exists on `connectdata->cselect_bits` where
  specific protocols can indicate something similar, only for the
  whole connection.
- `cselect_bits` are cleard in transfer.c on use and, importantly,
  also set when the transfer loop expended its `maxloops` tries.
  `drain` was not cleared by transfer and the http2/3 implementations
  had to take care of that.
- `dselect_bits` is cleared *and* set by the transfer loop. http2/3
  does no longer clear it, only set when new events happen.

This change unifies the handling of socket poll overrides, extending
`cselect_bits` by a easy handle specific value and a common treatment in
transfers.

Closes #11005