client.c: Replaced every "if (DEBUG)" by the new DEBUG macro

The new DEBUG macro simplifies the use of debug messages, using only one
line in code instead of the old if-then check followed by a printf.

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Acked-by: Ricardo Tavares de Oliveira <rto07@c3sl.ufpr.br>
Signed-off-by: Danilo K. S. Yorinori <danilok@c3sl.ufpr.br>