Make cli work on OpenBSD

[unkaktus]

cli does not build on OpenBSD anymore. This is the issue to track the progress on fixing related dependencies.

Steps to reproduce the issue:

1. GOOS=openbsd GOARCH=amd64 go get github.com/docker/cli/cmd/docker

Describe the results you received:
Various compile-time errors (non-impemented functions, missing syscalls etc)

Describe the results you expected:
Docker cli builds on OpenBSD.

Stages:

• Update golang.org/x/sys vendor moby/moby#36276
• Fix compilation of defaultCredentialStore() on unsupported platforms #872
• Generalize chtimes() implementation for non-linux platforms tonistiigi/fsutil#27
• Make sysx compile on OpenBSD; mark OpenBSD as unsupported for xattr containerd/continuity#104
• mount: add support for OpenBSD containerd/containerd#2712
• archive: add support for OpenBSD containerd/containerd#2713
• Add OpenBSD support for fs subpackage containerd/continuity#109
• update vendor in cli repo
• add openbsd/amd64 to cross build

[dnephin]

I added another task to the list. We should build it as part of cross so that we catch regressions.

[thaJeztah]

Moby changes are brought in through #892

[unkaktus]

updated the list as there are three more new blockers, ah.

[bsousaa]

Closing for now as there is no active work being done here. Though contributions are welcome.