'this-machine' and 'this-machine-ssh-configuration' were useless given
that there are no thunked fields.
* gnu/machine.scm (<machine>)[this-machine]: Remove.
* gnu/machine/ssh.scm (<machine-ssh-configuration>)
[this-machine-ssh-configuration]: Remove.
* gnu/machine.scm (roll-back-machine, &deploy-error, deploy-error?)
(deploy-error-should-roll-back)
(deploy-error-captured-args): New variable.
* gnu/machine/ssh.scm (roll-back-managed-host): New variable.
* guix/scripts/deploy.scm (guix-deploy): Roll-back systems when a
deployment fails.
* gnu/machine.scm (machine-system): Delete variable.
(machine-operating-system): New variable.
All callers changed.
* doc/guix.texi (Invoking guix deploy): Use the
'machine-operating-system' accessor rather than 'machine-system'.
