oauthutil: clarify token replacement prompt wording

The previous wording "Already have a token - refresh?" was misleading
because answering yes triggers a full re-authorization flow, not an
OAuth2 refresh token grant. Updated to "Token already configured -
replace it?" to accurately describe what happens.

Also updated the SugarSync backend which has its own copy of the prompt,
and the docs for box, drive, and onedrive that reference it.
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