Indeed, everything works much better if we agree to terminate header
lines with a newline rather than separate them.
## for each HEADER in the LIST which appears in the current message, where
## VALUE is the HEADER's current value.
m4_define(<:RENAME_HEADERS_ADD:>, <:m4_dnl
## for each HEADER in the LIST which appears in the current message, where
## VALUE is the HEADER's current value.
m4_define(<:RENAME_HEADERS_ADD:>, <:m4_dnl
${if def:h_$:><:1:{X-CONF_header_token-Renamed-$:><:1: $h_$:><:1:\n}}:>,
<:$1:>):>)
${if def:h_$:><:1:{X-CONF_header_token-Renamed-$:><:1: $h_$:><:1:\n}}:>,
<:$1:>):>)