windows: required environment variables

adds more of the undocumented-but-seemingly-required environment variables for uv_spawn

written to address https://github.com/JuliaLang/julia/issues/5574

this list was created with the help of https://github.com/Alexpux/Cygwin/blob/b266b04fbbd3a595f02ea149e4306d3ab9b1fe3d/winsup/cygwin/environ.cc#L955 to get a more complete list

I notice that cygwin file also ensures certain variables are uppercase for compatibility "for posix and backwards compatibility". Would you be interested in a pull request (written from scratch, but using the same variable list for reference) so that libuv also also does this? Or does this seem like a non-existent issue for libuv?