system: Add 'locale-libcs' field.
* gnu/system/locale.scm (localedef-command)[maybe-version-directory]:
New procedure.
Use it.
(locale-directory): Rename to...
(single-locale-directory): ... this. Check the version of LIBC to
determine whether to create a "X.Y" sub-directory or to make it a
symlink to ".". Add the version number in the derivation name.
(locale-directory): New procedure.
(%default-locale-libcs): New variable.
* gnu/system.scm (<operating-system>)[locale-libcs]: New field.
(operating-system-locale-directory): Pass it to 'locale-directory'.
* doc/guix.texi (operating-system Reference): Document 'locale-libcs'.
(Locales)[Locale Data Compatibility Considerations]: New section.