Doesn't build for plan9

[ncw]

This isn't a big deal for me, I'll just conditionally compile rclone for plan9 - thought you might like to know though!

$ GOOS=plan9 GOARCH=amd64 go build github.com/pkg/sftp
# github.com/pkg/sftp
go/src/github.com/pkg/sftp/attrs.go:90: undefined: fileStatFromInfoOs
go/src/github.com/pkg/sftp/attrs.go:186: undefined: syscall.S_ISGID
go/src/github.com/pkg/sftp/attrs.go:189: undefined: syscall.S_ISUID
go/src/github.com/pkg/sftp/attrs.go:192: undefined: syscall.S_ISVTX
go/src/github.com/pkg/sftp/attrs.go:219: undefined: syscall.S_ISGID
go/src/github.com/pkg/sftp/attrs.go:222: undefined: syscall.S_ISUID
go/src/github.com/pkg/sftp/attrs.go:225: undefined: syscall.S_ISVTX
go/src/github.com/pkg/sftp/server.go:56: undefined: syscall.EBADF
go/src/github.com/pkg/sftp/server.go:240: undefined: syscall.EBADF
go/src/github.com/pkg/sftp/server.go:555: undefined: syscall.Errno
go/src/github.com/pkg/sftp/server.go:240: too many errors

[eikenb]

If you'd like to submit a PR and help support plan9 as a platform, please do. Unfortunately I don't have the time right now to setup and learn plan9 in order to support it.

[ncw]

Fair enough! I don't use plan9 either, I just compile for it if possible.