guix package: Add '--do-not-upgrade' option.

* guix/scripts/package.scm (%options): Add the '--do-not-upgrade' option.
  (show-help): Document it.
  (options->installable): Add 'do-not-upgrade-regexps' variable.
  Use it in 'packages-to-upgrade'.
* doc/guix.texi (Invoking guix package): Document the '--do-not-upgrade'
  option.