Enable afero to be used on zos

[danmillwood]

Hello, Id like to contribute changes that enable afero to be used for go applications running on the zos operating system. The changes are all build tag related.

There are two commits: The first of these adds build tags to const_bsds.go and const_win_unix.go to enable afero to compile on zos. The second of these disables build of the sftpfs package files for zos. This is because github.com/pkg/sftp does not yet run on zos and this was causing unit test failures in afero.

I have run the unit tests on zos

:>go version
go version go1.16.5 zos/s390x
:>go clean --cache --modcache --testcache
:>go test ./...
go: downloading golang.org/x/text v0.3.3
ok  	github.com/spf13/afero	5.238s
ok  	github.com/spf13/afero/mem	0.357s
ok  	github.com/spf13/afero/tarfs	0.135s
ok  	github.com/spf13/afero/zipfs	0.140s

I've also run the unit tests on my Mac and they all pass

afero % go clean --cache --modcache --testcache
afero % go test ./...
go: downloading github.com/pkg/sftp v1.10.1
go: downloading golang.org/x/text v0.3.3
go: downloading golang.org/x/crypto v0.0.0-20190820162420-60c769a6c586
go: downloading github.com/pkg/errors v0.8.1
go: downloading github.com/kr/fs v0.1.0
ok  	github.com/spf13/afero	5.306s
ok  	github.com/spf13/afero/mem	0.582s
ok  	github.com/spf13/afero/sftpfs	6.445s
ok  	github.com/spf13/afero/tarfs	0.850s
ok  	github.com/spf13/afero/zipfs	0.503s

[CLAassistant]

All committers have signed the CLA.

[danmillwood]

Closing as this has now been added