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/security_windows.go

@@ -5,6 +5,7 @@
 package windows
 
 import (
+	"syscall"
 	"unsafe"
 )
 
@@ -123,7 +124,7 @@ func StringToSid(s string) (*SID, error) {
 // System specify target computer to search.
 func LookupSID(system, account string) (sid *SID, domain string, accType uint32, err error) {
 	if len(account) == 0 {
-		return nil, "", 0, EINVAL
+		return nil, "", 0, syscall.EINVAL
 	}
 	acc, e := UTF16PtrFromString(account)
 	if e != nil {