config: copy config URIs before a modification

A recent TLS options support patchset (PR #9385) has changed a way to
work with URIs: they're now tables, not strings. So, a modification may
influence the original data.

Prevent it by copying a table before a modification.

Part of #8862

NO_DOC=bugfix
NO_CHANGELOG=the fixed code was not released yet
NO_TEST=no known failure scenarios