1. For DEFAULT_CHARSET/OEM_CHARSET, use correct encoding
based on LibreOffice Default Language for Documents setting
(Tools->Options...->Language Settings->Languages).
For that, two functions added to tencinfo.h, that map language
names to corresponding Windows ANSI/OEM encodings.
2. If charset is DEFAULT_CHARSET/OEM_CHARSET for Symbol font,
then always use RTL_TEXTENCODING_SYMBOL.

Unit test is included.

Change-Id: Ibff63e7a03dec42a9d2a74399936d6bc04f2ff1a
Reviewed-on: https://gerrit.libreoffice.org/28322Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>