use g_date_time_format instead of eel_strdup_strftime (bug #647032, #647050)

g_date_time_format supports a wider range of format specifiers,
including some that eel_strdup_strftime didn't support but have been
used in translations for years (oops).

Also return "Never" instead of NULL in rb_utf_friendly_time, since
that isn't such an obviously incorrect case that it should be treated
as an error.  This fixes some weird crashes.