MINOR: debug: only emit and count warnings when explicitly requested

MINOR: debug: only emit and count warnings when explicitly requested

Previously complain() would set TAINTED_WARN was set even for a
check_if() because there was no way to distinguish a warn from a check
and a warn was systematically accounted for when the type was not a
bug. Likewise, BUG_ON() would dump a trace whenever a non-fatal event
happened, which is not the goal either as it prevents from counting
events without logging them (e.g. COUNT_IF). Now that we have explicit
flags for each type, let's consider them when choosing what to emit.

Note that for now complain() will still log the "msg" line if used
without FATL nor WARN though.