os: remove special casing of NUL in Windows file operations
Some file operations, notably Stat and Mkdir, special cased their
behavior when operating on a file named "NUL" (case-insensitive).
This check failed to account for the many other names of the NUL
device, as well as other non-NUL device files: "./nul", "//./nul",
"nul.txt" (on some Windows versions), "con", etc.
Remove the special case.
os.Mkdir("NUL") now returns no error. This is consonant with the
operating system's behavior: CreateDirectory("NUL") succeeds, as
does "MKDIR NUL" on the command line.
os.Stat("NUL") now follows the existing path for FILE_TYPE_CHAR devices,
returning a FileInfo which correctly reports the file as being a
character device.
os.Stat and os.File.Stat have common elements of their logic unified.