BUG/MEDIUM: fd: HUP is an error only when write is active

BUG/MEDIUM: fd: HUP is an error only when write is active

William reported that since commit 6b3089856f ("MEDIUM: fd: do not use
the FD_POLL_* flags in the pollers anymore") the master's CLI often
fails to access sub-processes. There are two causes to this. One is
that we did report FD_POLL_ERR on an FD as soon as FD_EV_SHUT_W was
seen, which is automatically inherited from POLLHUP. And since we do
not store the current shutdown state of an FD we can't know if the
poller reports a sudden close resulting from an error or just a
byproduct of a previous shutdown(WR) followed by a read0. The current
patch addresses this by only considering this when the FD was active,
since a shutdown FD is not active. The second issue is that *somewhere*
down the chain, channel data are ignored if an error is reported on a
channel. This results in content truncation, but this cause was not
figured yet.

No backport is needed.