go.sys/windows: delete errors_windows.go (except APPLICATION_ERROR) and Errno

All consts in errors_windows.go (except APPLICATION_ERROR) were
"invented" at the start of windows port to have minimal impact on
existing Go packages. No point keeping them around.
Also remove Errno, since we will be using syscall.Errno everywhere anyway.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/128290044

windows/syscall.go

@@ -21,13 +21,15 @@
 // holds a value of type syscall.Errno.
 package windows
 
+import "syscall"
+
 // ByteSliceFromString returns a NUL-terminated slice of bytes
 // containing the text of s. If s contains a NUL byte at any
-// location, it returns (nil, EINVAL).
+// location, it returns (nil, syscall.EINVAL).
 func ByteSliceFromString(s string) ([]byte, error) {
 	for i := 0; i < len(s); i++ {
 		if s[i] == 0 {
-			return nil, EINVAL
+			return nil, syscall.EINVAL
 		}
 	}
 	a := make([]byte, len(s)+1)
@@ -37,7 +39,7 @@ func ByteSliceFromString(s string) ([]byte, error) {
 
 // BytePtrFromString returns a pointer to a NUL-terminated array of
 // bytes containing the text of s. If s contains a NUL byte at any
-// location, it returns (nil, EINVAL).
+// location, it returns (nil, syscall.EINVAL).
 func BytePtrFromString(s string) (*byte, error) {
 	a, err := ByteSliceFromString(s)
 	if err != nil {