pack: Add '--profile-name'.

* guix/scripts/pack.scm (self-contained-tarball): Add #:profile-name and
honor it.
(squashfs-image, docker-image): Add #:profile-name.
(%default-options): Add 'profile-name'.
(%options, show-help): Add "--profile-name".
(guix-pack): Honor it.
* tests/guix-pack-localstatedir.sh: New file.
* Makefile.am (SH_TESTS): Add it.
* doc/guix.texi (Invoking guix pack): Document "--profile-name".