std.os.getenv and std.os.getenvZ have nice compile errors when not linking
libc and using Windows.
std.os.getenvW is provided as a Windows-only API that does not require
an allocator. It uses the Process Environment Block.
std.process.getEnvVarOwned is improved to be a simple wrapper on top of
std.os.getenvW.
std.process.getEnvMap is improved to use the Process Environment Block
rather than calling GetEnvironmentVariableW.
std.zig.system.NativePaths uses process.getEnvVarOwned instead of
std.os.getenvZ, which works on Windows as well as POSIX.
The actual desired access mask in this case seems quite confusing and badly documented. The previous combination of `GENERIC_READ` and `SYNCHRONIZE` seems both illegal and redundant according to the [`ntifs.h` documentation](https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/nf-ntifs-ntcreatefile), which specifies that `GENERIC_READ` should not be used for directories and includes `SYNCHRONIZE`. `winnt.h` contains a number of relevant-sounding flags such as `FILE_ADD_FILE`, `FILE_ADD_SUBDIRECTORY`, and `FILE_DELETE_CHILD` that do not show up in documentation at all. These are equal in value to file-specific flags that are documented as flags you should not specify when opening a directory.
