container: Pass a list of <file-system> objects as things to mount.

* gnu/build/linux-container.scm (mount-file-systems): 'mounts' is now a
list of <file-system> objects instead of a list of lists ("specs").
Add call to 'file-system->spec' as the argument to 'mount-file-system'.
(run-container, call-with-container): Adjust docstring accordingly.
* gnu/system/file-systems.scm (spec->file-system): New procedure.
* gnu/system/linux-container.scm (container-script)[script]: Call
'spec->file-system' inside gexp.
* guix/scripts/environment.scm (launch-environment/container): Remove
call to 'file-system->spec'.
* tests/containers.scm ("call-with-container, mnt namespace")
("call-with-container, mnt namespace, wrong bind mount"): Pass a list of
<file-system> objects.

tests/containers.scm

@@ -20,6 +20,7 @@
   #:use-module (guix utils)
   #:use-module (guix build syscalls)
   #:use-module (gnu build linux-container)
+  #:use-module (gnu system file-systems)
   #:use-module (srfi srfi-64)
   #:use-module (ice-9 match))
 
@@ -80,7 +81,10 @@
 (skip-if-unsupported)
 (test-assert "call-with-container, mnt namespace"
   (zero?
-   (call-with-container '(("none" device "/testing" "tmpfs" () #f #f))
+   (call-with-container (list (file-system
+                                (device "none")
+                                (mount-point "/testing")
+                                (type "tmpfs")))
      (lambda ()
        (assert-exit (file-exists? "/testing")))
      #:namespaces '(user mnt))))
@@ -91,8 +95,11 @@
   ;; An exception should be raised; see <http://bugs.gnu.org/23306>.
   (catch 'system-error
     (lambda ()
-      (call-with-container '(("/does-not-exist" device "/foo"
-                              "none" (bind-mount) #f #f))
+      (call-with-container (list (file-system
+                                   (device "/does-not-exist")
+                                   (mount-point "/foo")
+                                   (type "none")
+                                   (flags '(bind-mount))))
         (const #t)
         #:namespaces '(user mnt)))
     (lambda args