Win32: Unicode file name support (except dirent)

Replaces Windows "ANSI" APIs dealing with file- or path names with their
Unicode equivalent, adding UTF-8/UTF-16LE conversion as necessary.

The dirent API (opendir/readdir/closedir) is updated in a separate commit.

Adds trivial wrappers for access, chmod and chdir.

Adds wrapper for mktemp (needed for both mkstemp and mkdtemp).

The simplest way to convert a repository with legacy-encoded (e.g. Cp1252)
file names to UTF-8 ist to checkout with an old msysgit version and
"git add --all & git commit" with the new version.

Signed-off-by: Karsten Blees <blees@dcon.de>

Author: kblees