services: Don't use the deprecated 'make-forkexec-constructor' call.

Passing 'make-forkexec-constructor' a string or several string arguments
has been deprecated since dmd 0.1.

* gnu/services/base.scm (rngd-service-type): In 'start' method, pass a
list as the first argument to 'make-forkexec-constructor'.
* gnu/services/desktop.scm (bluetooth-shepherd-service): Likewise.
* gnu/services/spice.scm (spice-vdagent-shepherd-service): Likewise.

gnu/services/base.scm

@@ -557,7 +557,7 @@ (define rngd-command
         (documentation "Add TRNG to entropy pool.")
         (requirement '(udev))
         (provision '(trng))
-        (start #~(make-forkexec-constructor #$@rngd-command))
+        (start #~(make-forkexec-constructor '#$rngd-command))
         (stop #~(make-kill-destructor))))))
 
 (define* (rngd-service #:key

gnu/services/desktop.scm

@@ -452,8 +452,8 @@ (define (bluetooth-shepherd-service config)
    (requirement '(dbus-system udev))
    (documentation "Run the bluetoothd daemon.")
    (start #~(make-forkexec-constructor
-             (string-append #$(bluetooth-configuration-bluez config)
-                            "/libexec/bluetooth/bluetoothd")))
+             (list #$(file-append (bluetooth-configuration-bluez config)
+                                  "/libexec/bluetooth/bluetoothd"))))
    (stop #~(make-kill-destructor))))
 
 (define bluetooth-service-type

gnu/services/spice.scm

@@ -54,7 +54,7 @@ (define spice-vdagentd-command
       (documentation "Spice vdagentd service")
       (requirement '(udev))
       (provision '(spice-vdagentd))
-      (start #~(make-forkexec-constructor #$@spice-vdagentd-command))
+      (start #~(make-forkexec-constructor '#$spice-vdagentd-command))
       (stop #~(make-kill-destructor)))))
 
 (define spice-vdagent-profile