Don't run gpg when loading package.el

Don't run gpg when loading package.el

* lisp/emacs-lisp/package.el (package-check-signature): Don't run
gpg on startup, but just default to `allow-unsigned'.
(package-check-signature): New function to check whether a OpenPGP
configuration is found when `allow-unsigned'.
(package--check-signature-content, package--check-signature)
(package--download-one-archive, package-refresh-contents)
(package-install-from-archive): Use function instead of variable
throughout.
* doc/emacs/package.texi (Package Installation): Document this.