ui: Add package-description-string.

Provide support for Texinfo's markup in package description.

* guix/ui.scm (%text-width): New parameter.
  (texi->plain-text): New variable.
  (package->recutils): Use them.
  (package-description-string): New variable.
* emacs/guix-main.scm (%package-param-alist): Use it.
* gnu/packages/perl.scm (perl-devel-globaldestruction)
  (perl-devel-lexalias, perl-exporter-lite): Adapt to Texinfo's markup.
* gnu/packages/python.scm (python2-empy): Likewise.