config: remove unnecessary copy of config line's space prefix before printing

config: remove unnecessary copy of config line's space prefix before printing

When we wanted to insert '#' before a config line (to comment it out),
we used dm_pool_strndup to temporarily copy the space prefix first so
we can assemble the final line with:

   "<space_prefix># <key>=<value>":

out of original:

  "<space_prefix><key>=<value>".

The space_prefix copy is not necessary, we can just use fprintf's
precision modifier "%.*s" to print the exact part if we alrady
know space_prefix length.