system: Build system-wide locale definitions.

system: Build system-wide locale definitions.

* gnu/system/locale.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.
* gnu/system.scm (<operating-system>)[locale]: Change the default to
  "en_US.utf8".
  (operating-system-locale-directory): New procecure.
  (operating-system-derivation): Use it.
* doc/guix.texi (Using the Configuration System): Change example locale
  to "fr_FR.utf8".
  (operating-system Reference): Add xref to "Locales".  Document
  'locale-definitions'.
  (Locales): New section.
* po/guix/POTFILES.in: Add gnu/system.scm.