syscall: use error

- syscall (not os) now defines the Errno type.
- the low-level assembly functions Syscall, Syscall6, and so on
  return Errno, not uintptr
- syscall wrappers all return error, not uintptr.

R=golang-dev, mikioh.mikioh, r, alex.brainman
CC=golang-dev
https://golang.org/cl/5372080

root_windows.go

@@ -12,8 +12,8 @@ import (
 )
 
 func loadStore(roots *x509.CertPool, name string) {
-	store, errno := syscall.CertOpenSystemStore(syscall.InvalidHandle, syscall.StringToUTF16Ptr(name))
-	if errno != 0 {
+	store, err := syscall.CertOpenSystemStore(syscall.InvalidHandle, syscall.StringToUTF16Ptr(name))
+	if err != nil {
 		return
 	}