pack: Add '--symlink'.

* guix/scripts/pack.scm (self-contained-tarball): Add #:symlinks
parameter.
[build](symlink->directives): New procedure
(directives): New variable.
Add call to 'evaluate-populate-directive'.  Pass the directories among
DIRECTIVES to 'tar'.
(%default-options): Add 'symlinks'.
(%options, show-help): Add '--symlink'.
(guix-pack): Honor it.
* gnu/build/install.scm (evaluate-populate-directive): Export.
* doc/guix.texi (Invoking guix pack): Document it.