client.h: Added EXEFILE define to be used instead of the "const char *exe"

All other constant values are defines (changed to keep it standarded).

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Acked-by: Vinicius Ruoso <vkr07@c3sl.ufpr.br>
Signed-off-by: Danilo K. S. Yorinori <danilok@c3sl.ufpr.br>