MINOR: debug: make the complain() function start to complete the message

MINOR: debug: make the complain() function start to complete the message

The function will be extended to automatically prepend/append some info.
For now all it does is to always prepend/append the "\n" that each message
has at the beggining and at the end.

We're doing this into a locally allocated iovec and emit it at once using
writev(). This will be easy to extend to pass extra info. The \n alone
removed 1.5 kB of code.