Set default encodings on Windows

Default to utf-8 internally on Windows and use cp850 for terminal. Also set some default fileencoding heuristics that are likely to match not only Windows files but also standard encodings of the 2010s. This fixes at least two problems on Windows:
1. The Windows installer currently throws errors, because Python(?) doesn't work properly without proper encoding.
2. Ticket #340, which had problems with utf-8 listchars working properly unless they were re-set in .vimrc.local with the correct encoding.