Add SSHManager support for invoking Windows workers via cmd.exe

[mgkuhn]

Distributed.addprocs() now supports four new keyword arguments shell,
ssh, env and cmdline_cookie.

Specifying shell=:wincmd now makes it possible to start workers on a
Windows machine with an sshd server that invokes cmd.exe as the shell
(e.g. Microsoft's OpenSSH port does that by default). Previously
SSHManager only supported ssh connections to a POSIX shell.

Specifying ssh="/usr/bin/ssh" makes it possible to specify the ssh
client that SSHManager will use (useful for debugging and where a
custom-version of ssh is required).

The new env parameter now allows to pass arbitrary environment
variables to workers.

Specifying cmdline_cookie=true is a workaround for an ssh problem
with Windows workers that run older (pre-ConPTY) version of Windows,
Julia, or OpenSSH.

(This is a revised version of PR #30614)

[musm]

LGTM pending #38352

[jebej]

Is it possible to make this work for when ssh conects to powershell instead of cmd? Maybe by first starting cmd.exe?

[mgkuhn]

Adding PowerShell command-invocation support is on my todo list, but will be a future, separate PR. (Proper meta-character escaping in PowerShell sadly can be even more complicated than in cmd.exe. Neither has an escaping mechanism as simple and clean as we are used to from the POSIX shell.)

[jebej]

So it wouldn't be possible to just start cmd.exe from powershell and then use the work from this PR?

[musm]

Looks like this needs a rebase to remove the cmd.exe escaping functions since we've merged #38352

[mgkuhn]

Rebased after merge of #38352

[musm]

Failures look unrelated.
One is a linear algebra failure: https://build.julialang.org/#/builders/52/builds/5721/steps/5/logs/stdio
The other is Profile error: https://build.julialang.org/#/builders/4/builds/6152/steps/5/logs/stdio

Will merge tomorrow sans objections.