* guix/scripts/pack.scm (%options): Extract symlink parsing logic to...
(symlink-spec-option-parser): ... here.
(self-contained-tarball/builder): Add a comment mentioning why a relative file
name is used for the link target.
* guix/scripts/environment.scm (show-environment-options-help): Document new
--symlink option.
(%default-options): Add default value for symlinks.
(%options): Register new symlink option.
(launch-environment/container): Add #:symlinks argument and extend doc, and
create symlinks using evaluate-populate-directive.
(guix-environment*): Pass symlinks arguments to launch-environment/container.
* doc/guix.texi (Invoking guix shell): Document it.
* tests/guix-shell.sh: Add a --symlink (negative) test.
* tests/guix-environment-container.sh: Add tests.