Allow using locales when gettext is absent

Allow using locales when gettext is absent

Previously, if configure did not detect dgettext(), we disabled
anything that smelled like localization, inadvertently including
setlocale().  Now that we use setlocale(LC_ALL, ""), we have
localized dates available as well as messages, so we should not
disable calls to setlocale() any more.
Since the routines from locale.h are only used in a relatively
small number of places, just include the header directly in those
files and remove it from k5-platform.h.