fix linux

* 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)

std/unicode.zig

@@ -495,7 +495,7 @@ pub fn utf16leToUtf8Alloc(allocator: *mem.Allocator, utf16le: []const u16) ![]u8
 }
 
 /// Asserts that the output buffer is big enough.
-/// Returns end index.
+/// Returns end byte index into utf8.
 pub fn utf16leToUtf8(utf8: []u8, utf16le: []const u16) !usize {
     var end_index: usize = 0;
     var it = Utf16LeIterator.init(utf16le);