Allow processing of common options via FlagSet

[abergmeier]

What type of PR is this?

/kind api-change
/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test

/kind feature

/kind flake
/kind other

What this PR does / why we need it:

Am using Buildah - but don't want to use Cobra.

How to verify it

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

None

[TomSweeneyRedHat]

@abergmeier thanks for the change, but our CI isn't thrilled with it...

[+0071s] # github.com/containers/buildah/pkg/parse
[+0071s] pkg/parse/parse.go:61:39: undefined: pflag
[+0071s] pkg/parse/parse.go:122:19: undefined: c
[+0071s] pkg/parse/parse.go:128:35: undefined: c
[+0071s] pkg/parse/parse.go:135:18: undefined: c
[+0071s] pkg/parse/parse.go:136:17: undefined: c
[+0071s] pkg/parse/parse.go:137:18: undefined: c
[+0071s] pkg/parse/parse.go:138:18: undefined: c
[+0071s] pkg/parse/parse.go:141:5: undefined: flag
[+0071s] pkg/parse/parse.go:152:17: undefined: c
[+0071s] pkg/parse/parse.go:153:17: undefined: c
[+0071s] pkg/parse/parse.go:153:17: too many errors

[rhatdan]

LGTM
@TomSweeneyRedHat @vrothberg @nalind @saschagrunert PTAL

[vrothberg]

/lgtm

[rhatdan]

/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: abergmeier, rhatdan

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