file-systems: Add '%network-configuration-files' and '%network-file-mappings'.

* gnu/system/file-systems.scm (%network-configuration-files)
(%network-file-mappings): New variables.
* guix/scripts/environment.scm (%network-configuration-files): Remove.
(launch-environment/container): Refer to '%network-file-mappings'
instead of calling 'filter-map'.