guix home: Add 'container' command.

guix home: Add 'container' command.

* guix/scripts/home.scm (show-help, %options): Add '--network',
'--share', and '--expose'.
(not-config?, user-shell, spawn-home-container): New procedures.
(%default-system-profile): New variable.
(perform-action): Add #:file-system-mappings, #:container-command,
and #:network?; honor them.
(process-action): Adjust accordingly.
(guix-home)[parse-sub-command]: Add "container".
[parse-args]: New procedure.
Use it instead of 'parse-command-line'.
* tests/guix-home.sh: Add tests.
* doc/guix.texi (Declaring the Home Environment): Mention 'guix home
container' as a way to test configuration.
(Invoking guix home): Document it.