The set of errors forwarded by the os package varied with system and
was therefore non-portable.
Three helpers added for portable error checking: IsExist, IsNotExist, and IsPermission.
One or two more may need to come, but let's keep the set very small to discourage
thinking about errors that way.
R=mikioh.mikioh, gustavo, r, rsc
CC=golang-dev
https://golang.org/cl/5672047
(more are possible but omitted for now as they are part of
specific tests where rather than changing what is there we
should probably expand the tests to cover the new case)
R=rsc, dvyukov
CC=golang-dev
https://golang.org/cl/5247058
Make plan 9 Readdir & Readdirnames return os.EOF at end.
Also fix typos in the unix and windows comments.
R=golang-dev, fshahriar, bradfitz, rsc, r
CC=golang-dev
https://golang.org/cl/4557053
