Use nl_langinfo() in wxUILocaleImplUnix::GetInfo()

This function can be used for all GetInfo() items, so using it is
simpler than the code in the Unix version of wxLocale::GetInfo() which
uses either it or localeconv(), and there should be no real drawbacks to
using it nowadays as it should be available everywhere.

No real changes yet.

include/wx/unix/private/uilocale.h

@@ -30,4 +30,7 @@ inline wxString ExtractNotLang(const wxString& langFull)
 
 const char *wxSetlocaleTryAll(int c, const wxString& lc);
 
+// Extract date format from D_T_FMT value.
+wxString wxGetDateFormatOnly(const wxString& fmt);
+
 #endif // _WX_UNIX_PRIVATE_UILOCALE_H_