bug#17653: 24.3; Tramp doesn't detect ssh ControlMaster

[Juliusz Chroboczek <jch@pps.univ-paris-diderot.fr>]

>> The issue is that tramp-detect-ssh-controlmaster looks for the fixed
>> string "Missing ControlMaster argument", while my version of ssh returns
>> a differently worded error message.

> The native way to fix this in Tramp is tweaking `tramp-methods'. Add a
> new method "myssh" with your preferred arguments.

I know I can tweak it -- that's Emacs, I can advise any function -- but
that's not the issue.  I expect Emacs to work optimally out of the box on
an up-to-date Debian testing box, and this is not the case.

Ideally, tramp should use a less fragile method to check for ControlMaster
support; if that's not possible, it should at least use a regexp that's
permissive enough to work on recent OpenSSH.

Please fix Emacs to detect ControlMaster out of the box with recent Debian.

-- Juliusz