Bug#1117574: Acknowledgement (openssh-client: output pauses in interactive session over ControlMaster)

A little more info: I can reproduce the problem when building the
upstream/10.1p1 tag from salsa's ssh-team/openssh.git repository.
Bisecting between that and upstream/10.0p1 turns up 289239046 (upstream:
Make ssh(1) and sshd(8) set IP QoS (aka IP_TOS, IPV6_TCLASS),
2025-08-18) as the culprit.

(If anybody else wants to bisect, note that you'll need to "cherry-pick
-n b7ee13fbbb4eba" to get the guilty commit and some of its neighbors to
compile).

Scanning upstream commits from GitHub's openssh/openssh-portable
repository, it looks like this may have been noticed and fixed upstream
already via beae06f56 (upstream: don't reuse c->isatty for signalling
that the remote channel, 2025-10-07). And indeed, cherry-picking that
patch on top of 10.1p1 seems to make the problem go away for me.

-Peff