config: use the new '_unset' value for 'configwith'
This should let 'configwith' delegate all special processing of the default
config value to the main 'config' method.
This changeset introduce a small change in behavior since the default value is
run through the 'convert' function. This does not seems harmful and no actual
test break. This small change make the code simpler so I'm keeping it.
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