system: Selected locale is automatically built.

Fixes <http://bugs.gnu.org/22572>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/system/locale.scm (%not-dot): New variable.
(denormalize-codeset, locale-name->definition): New procedures.
* gnu/system.scm (locale-name->definition*): New procedure.
(operating-system-locale-directory): Instead of raising an error, add
the missing locale.
* doc/guix.texi (Locales): Adjust accordingly.