Fix tar package handling, and clean up package-subdirectory-regexp usage.

Fix tar package handling, and clean up package-subdirectory-regexp usage.

* lisp/startup.el (package-subdirectory-regexp): Move from package.el.
Omit \\` and \\', and let callers add them.

* lisp/emacs-lisp/package.el (package-strip-version)
(package-load-all-descriptors): Add \\` and \\' to
package-subdirectory-regexp before using it.
(package-untar-buffer): New arg DIR; ensure that file untars only
into this expected directory.  Remove superfluous delete-region.
(package-unpack): Caller changed.
(package-tar-file-info): Use package-subdirectory-regexp.