MEDIUM: tools: add csv_enc_append() to preserve the original chunk

MEDIUM: tools: add csv_enc_append() to preserve the original chunk

We have csv_enc() but there's no way to append some CSV-encoded data
to an existing chunk, so here we modify the existing function for this
and create an inlined version of csv_enc() which first resets the output
chunk. It will be handy to append data to an existing chunk without
having to use an extra temporary chunk, or to encode multiple strings
into a single chunk with chunk_newstr().

The patch is quite small, in fact most changes are typo fixes in the
comments.