* gnu/machine.scm: Fix typo in top-level comment.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

* guix/utils.scm (<location>, source-properties->location)
(location->source-properties, &error-location): Move to...
* guix/diagnostics.scm: ... here.
* gnu.scm: Adjust imports accordingly.
* gnu/machine.scm: Likewise.
* gnu/system.scm: Likewise.
* gnu/tests.scm: Likewise.
* guix/inferior.scm: Likewise.
* tests/channels.scm: Likewise.
* tests/packages.scm: Likewise.

As requested here:
<https://lists.gnu.org/archive/html/guix-devel/2020-02/msg00128.html>.

* .mailmap: Add an entry for Jakob.
* gnu/machine.scm, gnu/machine/digital-ocean.scm, gnu/machine/ssh.scm,
gnu/packages/admin.scm, gnu/packages/i2p.scm, gnu/packages/music.scm,
gnu/packages/web.scm, gnu/tests/reconfigure.scm, guix/scripts/deploy.scm,
guix/scripts/system/reconfigure.scm: Update their e-mail address.

'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 (<machine>): Rename accessor to
'%machine-operating-system'.
(machine-operating-system): New procedure.
* doc/guix.texi (Service Reference): Mention it.

* 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'.

* gnu/machine.scm: New file.
* gnu/machine/ssh.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.