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/exec_windows.go

@@ -8,6 +8,7 @@ package windows
 
 import (
 	"sync"
+	"syscall"
 	"unicode/utf16"
 	"unsafe"
 )
@@ -149,7 +150,7 @@ func getFullPath(name string) (path string, err error) {
 			return "", err
 		}
 		if n > uint32(len(buf)) {
-			return "", EINVAL
+			return "", syscall.EINVAL
 		}
 	}
 	return UTF16ToString(buf[:n]), nil
@@ -166,7 +167,7 @@ func normalizeDir(dir string) (name string, err error) {
 	}
 	if len(ndir) > 2 && isSlash(ndir[0]) && isSlash(ndir[1]) {
 		// dir cannot have \\server\share\path form
-		return "", EINVAL
+		return "", syscall.EINVAL
 	}
 	return ndir, nil
 }
@@ -180,7 +181,7 @@ func volToUpper(ch int) int {
 
 func joinExeDirAndFName(dir, p string) (name string, err error) {
 	if len(p) == 0 {
-		return "", EINVAL
+		return "", syscall.EINVAL
 	}
 	if len(p) > 2 && isSlash(p[0]) && isSlash(p[1]) {
 		// \\server\share\path form
@@ -189,7 +190,7 @@ func joinExeDirAndFName(dir, p string) (name string, err error) {
 	if len(p) > 1 && p[1] == ':' {
 		// has drive letter
 		if len(p) == 2 {
-			return "", EINVAL
+			return "", syscall.EINVAL
 		}
 		if isSlash(p[2]) {
 			return p, nil
@@ -217,7 +218,7 @@ func joinExeDirAndFName(dir, p string) (name string, err error) {
 		}
 	}
 	// we shouldn't be here
-	return "", EINVAL
+	return "", syscall.EINVAL
 }
 
 type ProcAttr struct {
@@ -238,7 +239,7 @@ var zeroSysProcAttr SysProcAttr
 
 func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle uintptr, err error) {
 	if len(argv0) == 0 {
-		return 0, 0, EWINDOWS
+		return 0, 0, syscall.EWINDOWS
 	}
 	if attr == nil {
 		attr = &zeroProcAttr
@@ -249,7 +250,7 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 	}
 
 	if len(attr.Files) > 3 {
-		return 0, 0, EWINDOWS
+		return 0, 0, syscall.EWINDOWS
 	}
 
 	if len(attr.Dir) != 0 {
@@ -337,5 +338,5 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 }
 
 func Exec(argv0 string, argv []string, envv []string) (err error) {
-	return EWINDOWS
+	return syscall.EWINDOWS
 }