This adds a new mechanism for logging, intended to replace debug().
The entry points are FLOG and FLOGF. FLOG can be used to log a sequence of
arguments, FLOGF is for printf-style formatted strings.
Each call to FLOG and FLOGF requires a category. If logging for a category
is not enabled, there is no effect (and arguments are not evaluated).
Categories may be enabled on the command line via the -d option.