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