make ssh connections via paramiko more configurable

[Antoine Martin]

OpenSSH supports configuration options, both in its config file and using the command line.
We should provide a similar way of setting options:

• reading the openssh ssh_config if present
• handling options in URL strings

See also:

• #2097 SSHFP with paramiko
• #2282 honour ssh config IdentityFile
• #1937 paramiko support for .ssh/config

[Antoine Martin]

Done in r25103.

Self-explanatory examples (add --debug=ssh for debugging):

xpra attach ssh://localhost/ --ssh=paramiko:verify-hostkey=no,auth=none+password

xpra attach ssh://localhost/ --ssh=paramiko:verify-hostkey=yes,verifyhostkeydns=yes,auth=agent+key

xpra attach ssh://localhost/ --ssh=paramiko:verifyhostkeydns=no,auth=password,numberofpasswordprompts=5

[Antoine Martin]

Let's also make it possible to specify the keyfile to use.

[Antoine Martin]

As of r25180, we can also specify paramiko attributes on the URI, including the key:

XPRA_SSH_DEFAULT_KEYFILES="" xpra attach "ssh://localhost/?key=$HOME/.ssh/id_rsa&auth=key"

The same options can still be specified as a general paramiko attribute:

XPRA_SSH_DEFAULT_KEYFILES="" xpra attach "ssh://localhost/" --ssh="paramiko:key=/home/antoine/.ssh/id_rsa,auth=key"

The new XPRA_SSH_DEFAULT_KEYFILES env var is used in these examples to disable the default ssh key locations.

Important: the old URI format ssh:host/DISPLAY is no longer supported. Parsing such strings was becoming increasingly unwieldy.

See also #2582.

[Antoine Martin]

r25180 caused a regression when there is no host config, fixed in r25214

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2567