* guix/scripts/system.scm (specification->file-system-mapping): Move from
here...
* guix/ui.scm (specification->file-system-mapping): ... to here.
* guix/scripts/enviroment.scm (show-help): Show help for new options.
(%options): Add --container --network, --expose, and --share options.
(%network-configuration-files): New variable.
(launch-environment, launch-environment/container, requisites*,
inputs->requisites): New procedures.
(guix-environment): Spawn new process in a container when requested.
* doc/guix.texi (Invoking guix environment): Document it.
* tests/guix-environment-container.sh: New file.
* Makefile.am (SH_TESTS): Add it.
