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

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.