[3006.x] Updated semanage fcontext to use --modify if context already exists when adding context

[dmurphy18]

What does this PR do?

If a semanage fcontext already exists for a file, then the command semanage fcontext --modify is used rather than semanage fcontext --add

What issues does this PR fix or reference?

Fixes:#64625

Previous Behavior

If the file already had a fcontext, a ValueError would be thrown, similar to the following

Unable to manage file: Problem setting fcontext: {'pid': 43053, 'retcode': 1, 'stdout': '', 'stderr': 'ValueError: File context for /root/test-one.txt already defined'}

New Behavior

The code checks to see if If the file already has a fcontext, and uses --modify rather than --add which will throw a ValueError if used in this case.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

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Yes

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