MEDIUM: debug: use \x1e (RS) to delimit the condition from vaargs in complain()

MEDIUM: debug: use \x1e (RS) to delimit the condition from vaargs in complain()

This is the same principle as for DBG_COUNT() but applied to complain().
For now it doesn't bring any particular benefit but we needed to know where
the end of the string is in order to continue to turn other fields to
variables.