tool_xattr: create the user.creator xattr attribute

This indicates that the file was created by curl which can help a user
determine the origin of a file. Like the other attributes, this

tool_xattr: create the user.creator xattr attribute

This indicates that the file was created by curl which can help a user
determine the origin of a file. Like the other attributes, this is only
enabled with the --xattr option.

Closes #15237

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tests: sync feature names with `curl -V`

Some feature names used in tests had minor differences compared to
the well-known ones from `curl -V`. This patch syncs them to make test
res

tests: sync feature names with `curl -V`

Some feature names used in tests had minor differences compared to
the well-known ones from `curl -V`. This patch syncs them to make test
results easier to grok.

Closes #14183

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test687/688: two more basic --xattr tests

Closes #11697