If we use a local type, it won't compare properly with errors from
the rest of the standard library. Errors are the one type from syscall
that propagates through the system, so it's important to have only
one type for them.
Will apply the corresponding change to the other packages once
this is approved.

Also delete some deprecated code. This is a new package; no need
to carry the past.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/126250043