environment: Add --user.

This change allows overriding the home directory of all filesystem mappings to
help hide the identity of the calling user in a container.

* doc/guix.texi (Invoking guix environment)[--container]: Mention --user.
[--user]: Add item.
* guix/scripts/environment.scm (show-help): Add --user.
(%options): Add --user.
(launch-environment/container) Add 'user' parameter.  Update doc.  Override
'user-mappings' using 'override-user-mappings'.  Consider override for chdir.
(mock-passwd, user-override-home, overrid-euser-dir): New procedures.
(guix-environment): Disallow --user without --container.  Provide user to
'launch-environment/container'.
* tests/guix-environment.sh: Add user test.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>