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.
This adds an option --print-rusage-self to the fish executable. When set,
this option prints some getrusage stats to the console in a human-readable
way. This will be used by upcoming benchmarking support.
