BUG/MEDIUM: polling: ensure we update FD status when there's no more activity

Some rare unexplained busy loops were observed on versions up to 1.5-dev19.
It happens that if a file descriptor happens to be disabled for both read and
write while it was speculatively enabled for both and this without creating a
new update entry, there will be no way to remove it from the speculative I/O
list until some other changes occur. It is suspected that a double sequence
such as enable_both/disable_both could have led to this situation where an
update cancels itself and does not clear the spec list in the poll loop.
While it is unclear what I/O sequence may cause this situation to arise, it
is safer to always add the FD to the update list if nothing could be done on
it so that the next poll round will automatically take care of it.

This is 1.5-specific, no backport is needed.
MEDIUM: listener: fix polling management in the accept loop

The accept loop used to force fd_poll_recv() even in places where it
was not completely appropriate (eg: unexpected errors). It does not
yet cause trouble but will do with the upcoming polling changes. Let's
use it only where relevant now. EINTR/ECONNABORTED do not result in
poll() anymore but the failed connection is simply skipped (this code
dates from 1.1.32 when error codes were first considered).
MINOR: protocol: improve the proto->drain() API

It was not possible to know if the drain() function had hit an
EAGAIN, so now we change the API of this function to return :
  < 0 if EAGAIN was met
  = 0 if some data remain
  > 0 if a shutdown was received
MINOR: connection: add a new conn_drain() function

Till now there was no way to know from a connection if a previous
call to drain() had done any change. This function is used to drain
incoming data and to update the connection's flags at the same time.
It also correctly sets the polling flags on the connection if the
drain function indicates inability to receive. This function will
be used preferably over ctrl->drain() when a connection is used.
MEDIUM: tcp: report in tcp_drain() that lingering is already disabled on close

When an incoming shutdown or error is detected, we know that we
can safely close without disabling lingering. Do it in tcp_drain()
so that we don't have to do it from each and every caller.
MEDIUM: connection: update callers of ctrl->drain() to use conn_drain()

Now we can more safely rely on the connection state to decide how to
drain and what to do when data are drained. Callers don't need to
manipulate the file descriptor's state anymore.

Note that it also removes the need for the fix ea90063 ("BUG/MEDIUM:
stream-int: fix the keep-alive idle connection handler") since conn_drain()
correctly sets the polling flags.