* error.BadFd is not a valid error code. it would always be a bug to
get this error code.
* merge error.Io with existing error.InputOutput
* merge error.PathNotFound with existing error.FileNotFound.
Not all OS's support both.
* add os.File.openReadC
* add error.BadPathName for windows file operations with invalid
characters
* add os.toPosixPath to help stack allocate a null terminating byte
* add some TODOs for other functions to investigate removing the
allocator requirement
* optimize some implementations to use the alternate functions when
a null byte is already available
* add a missing error.SkipZigTest
* os.selfExePath uses a non-allocating API
* os.selfExeDirPath uses a non-allocating API
* os.path.real uses a non-allocating API
* add os.path.realAlloc and os.path.realC
* convert many windows syscalls to use the W versions (See #534)
Andrew Kelley