lua/js utils.get_env_list() uses `environ' which was ANSI, thus
it broke any unicode names/values.
mpv already has an internal utf8_environ for win32, but it's used
only at the getenv(..) wrapper and not exposed in itself, and also it
has lazy initialization - on first getenv() call.
Now `environ' maps to a function which ensures initialization while
keeping it an l-value (like posix expects).
The cost of this fuglyness is that files should include osdep/io.h
(which now declares environ as extern) rather than declaring it
themselves, or else the build will break on mingw.
