The SSH connection sharing function in Cygwin's openssh release, using
`ssh -M` or the ssh_config.5 ControlMaster configuration, doesn't appear
to work.  Setting up a master connection seems fine, but subsequent SSH
sessions that should reuse master connection instead set up their own
session, with an error stating "mux_client_request_session: read from
master failed: Connection reset by peer".

Simple testcase attached, which creates a master connection, starts a
long sleep in a second SSH session, then terminates the master
connection.  When I run this on one of my Debian boxes, the script takes
about 6s to run, as tearing down the master connection terminates the
long-running command.  When run from Cygwin, however, the script takes
about 64s, as the second SSH session creates an independent connection
that's unaffected by the master connection terminating.

On Debian:

```
$ time bash test.sh adam@192.168.1.131
Master running (pid=7008)

Exit request sent.

real    0m6.048s
user    0m0.083s
sys     0m0.111s
```

On Cygwin:

```
$ time bash test.sh adam@192.168.1.131
Master running (pid=5079)
mux_client_request_session: read from master failed: Connection reset by peer

Exit request sent.
finishing

real    1m4.071s
user    0m0.106s
sys     0m0.540s
```

Slightly redacted `cygcheck -srv` output also attached.

Adam