format: Add specifier ~h for localized number output.

format: Add specifier ~h for localized number output.

* doc/ref/misc-modules.texi (Formatted Output): Document ~h.  Recommend
  use of ~h instead of ~:d.

* module/ice-9/format.scm (format): Add support for ~h.

* test-suite/tests/format.test ("~h localized number"): New test prefix.

* test-suite/tests/i18n.test (%american-english-locale-name,
  %american-english-locale): New variables.
  (under-american-english-locale-or-unresolved): New procedure.
  ("format ~h"): New test prefix.