machine: Rename 'system' field.

* 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'.

doc/guix.texi

@@ -25510,7 +25510,7 @@ evaluates to.  As an example, @var{file} might contain a definition like this:
             %base-services))))
 
 (list (machine
-       (system %system)
+       (operating-system %system)
        (environment managed-host-environment-type)
        (configuration (machine-ssh-configuration
                        (host-name "localhost")
@@ -25551,7 +25551,7 @@ This is the data type representing a single machine in a heterogeneous Guix
 deployment.
 
 @table @asis
-@item @code{system}
+@item @code{operating-system}
 The object of the operating system configuration to deploy.
 
 @item @code{environment}

gnu/machine.scm

@@ -34,7 +34,7 @@
             machine?
             this-machine
 
-            machine-system
+            machine-operating-system
             machine-environment
             machine-configuration
             machine-display-name
@@ -85,14 +85,14 @@
   make-machine
   machine?
   this-machine
-  (system        machine-system)       ; <operating-system>
+  (operating-system machine-operating-system) ; <operating-system>
   (environment      machine-environment)      ; symbol
   (configuration    machine-configuration     ; configuration object
                     (default #f)))            ; specific to environment
 
 (define (machine-display-name machine)
   "Return the host-name identifying MACHINE."
-  (operating-system-host-name (machine-system machine)))
+  (operating-system-host-name (machine-operating-system machine)))
 
 (define (machine-remote-eval machine exp)
   "Evaluate EXP, a gexp, on MACHINE. Ensure that all the elements EXP refers to

gnu/machine/ssh.scm

@@ -166,7 +166,7 @@ of MACHINE's system profile, ordered from most recent to oldest."
 environment type of 'managed-host."
   (maybe-raise-unsupported-configuration-error machine)
   (mlet %store-monad ((boot-parameters (machine-boot-parameters machine)))
-    (let* ((os (machine-system machine))
+    (let* ((os (machine-operating-system machine))
            (eval (cut machine-remote-eval machine <>))
            (menu-entries (map boot-parameters->menu-entry boot-parameters))
            (bootloader-configuration (operating-system-bootloader os))