On Windows: convert _tzname[] strings from the system charset to UTF8 before use.

On Windows: convert _tzname[] strings from the system charset to UTF8 before use.

 In the Windows C Runtime the strings in the global array _tzname[]
   are encoded using the "system default ansi code page".
   They must be converted to UTF8 before use by Wireshark/GTK.
   (See comment in the code for details).

   Fixes Bug #5421 (I hope).
   https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=5421

Also: Fix bug: when the timezone name is not available from the system
      ?ST was used when ?DT should have been used (and vice-versa).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@35086 f5534014-38df-0310-8fa8-9805f1628bb7