Add support for /etc/ssh/ssh_config

[BillyCroan]

Is your feature request related to a problem? Please describe.
xpra does not use the right settings to connect to my predefined ssh hosts in /etc/ssh/ssh_config

Describe the solution you'd like
Utilize paramiko's support for system level configs. (/etc/ssh/ssh_config)

I think the fix would go here: https://github.com/Xpra-org/xpra/blob/master/xpra/net/ssh/paramiko_client.py#L213
but I'm not sure. I don't understand why it's under "nogssapi context"

Describe alternatives you've considered
I could symlink $HOME/.ssh/config to /etc/ssh/ssh_config but that might break other users of ssh. Maybe put ssh_config in /opt/ and link every user's to it, but I suspect ssh will complain about ownership of that file. probably would have a problem with the symlink into /etc/ssh as well come to think of it

Another workaround is using xpra with --ssh="/usr/bin/ssh" but I understand it is your intention to use paramiko instead of openssh, so I am requesting this support for global configs.

Additional context
I used to use just $HOME/.ssh/config, but it was inneficient and a source of errors to maintain /root/.ssh/config and other user .ssh configs in parallel and ssh has a global config area in /etc/ssh so I moved my ssh config to that.

Everything else worked. xpra didn't.

Paramiko looks like they already support system-level config but they have to be told to use it:
paramiko/paramiko#1748 (comment) |
paramiko/paramiko#363 |

[totaam]

nogssapi was needed before: #1646 (comment)
4b50a25 removes this workaround, we'll see if anything breaks.
Hopefully, nothing. I believe that we have removed support for the distributions that were shipping outdated paramiko versions with this compatibility bug.

As for parsing the global config, 1660cdf should do that.
Untested, let me know if that works or you - please post the -d ssh debug log if not.