environment: Add '--inherit'.

* guix/scripts/environment.scm (purify-environment): Add 'white-list'
parameter and honor it.
(create-environment): Add #:white-list parameter and honor it.
(launch-environment): Likewise.
(launch-environment/fork): Likewise.
(show-help, %options): Add '--inherit'.
(guix-environment): Define 'white-list' and pass it to
'launch-environment/fork'.
* tests/guix-environment.sh: Test '--inherit'.
* doc/guix.texi (Invoking guix environment): Document it.